[common] Fix NullPointerExceptions on null transform inputs

paimon-common/src/main/java/org/apache/paimon/predicate/StringTransform.java

@@ -158,7 +158,7 @@ public int hashCode() {
     @Override
     public String toString() {
         List<String> inputs =
-                this.inputs.stream().map(Object::toString).collect(Collectors.toList());
+                this.inputs.stream().map(String::valueOf).collect(Collectors.toList());
         return name() + "(" + String.join(", ", inputs) + ')';
     }
 }

paimon-common/src/main/java/org/apache/paimon/predicate/SubstringTransform.java

@@ -71,6 +71,10 @@ public final Object transform(InternalRow row) {
         if (begin instanceof FieldRef) {
             FieldRef beginRef = (FieldRef) begin;
             checkArgument(beginRef.type().is(INTEGER_NUMERIC));
+            // getInt on a null reads an undefined value on columnar rows
+            if (row.isNullAt(beginRef.index())) {
+                return null;
+            }
             beginIndex = row.getInt(beginRef.index());
         } else {
             beginIndex = Integer.parseInt(inputs.get(1).toString());
@@ -85,6 +89,9 @@ public final Object transform(InternalRow row) {
             if (end instanceof FieldRef) {
                 FieldRef endRef = (FieldRef) inputs.get(2);
                 checkArgument(endRef.type().is(INTEGER_NUMERIC));
+                if (row.isNullAt(endRef.index())) {
+                    return null;
+                }
                 endIndex = beginIndex + row.getInt(endRef.index()) - 1;
             } else {
                 endIndex = beginIndex + Integer.parseInt(inputs.get(2).toString()) - 1;

paimon-common/src/main/java/org/apache/paimon/predicate/TrimTransform.java

@@ -51,7 +51,14 @@ public BinaryString transform(List<BinaryString> inputs) {
             return null;
         }
         String sourceString = inputs.get(0).toString();
-        String charsToTrim = inputs.size() == 1 ? " " : inputs.get(1).toString();
+        String charsToTrim = " ";
+        if (inputs.size() == 2) {
+            if (inputs.get(1) == null) {
+                // StringUtils.ltrim/rtrim treat a null charsToTrim as a null result
+                return null;
+            }
+            charsToTrim = inputs.get(1).toString();
+        }
         switch (trimFlag) {
             case BOTH:
                 return BinaryString.fromString(StringUtils.trim(sourceString, charsToTrim));

paimon-common/src/test/java/org/apache/paimon/predicate/ConcatTransformTest.java

@@ -72,4 +72,13 @@ public void testConcatHybridInputs() {
                                 BinaryString.fromString("-he")));
         assertThat(result).isEqualTo(BinaryString.fromString("ha-he"));
     }
+
+    @Test
+    public void testToStringWithNullInput() {
+        List<Object> inputs = new ArrayList<>();
+        inputs.add(BinaryString.fromString("a"));
+        inputs.add(null);
+
+        assertThat(new ConcatTransform(inputs).toString()).isEqualTo("CONCAT(a, null)");
+    }
 }

paimon-common/src/test/java/org/apache/paimon/predicate/SubstringTransformTest.java

@@ -100,6 +100,27 @@ public void testSubstringRefInputs() {
         assertThat(result).isEqualTo(BinaryString.fromString("ell"));
     }
 
+    @Test
+    public void testNullPositionFieldYieldsNull() {
+        List<Object> inputs = new ArrayList<>();
+        inputs.add(new FieldRef(0, "f0", DataTypes.STRING()));
+        inputs.add(new FieldRef(1, "f1", DataTypes.INT()));
+        assertThat(
+                        new SubstringTransform(inputs)
+                                .transform(
+                                        GenericRow.of(
+                                                BinaryString.fromString("123-45-6789"), null)))
+                .isNull();
+
+        inputs.add(new FieldRef(2, "f2", DataTypes.INT()));
+        assertThat(
+                        new SubstringTransform(inputs)
+                                .transform(
+                                        GenericRow.of(
+                                                BinaryString.fromString("123-45-6789"), 8, null)))
+                .isNull();
+    }
+
     @Test
     public void testSubstringRefInputUsesSourceFieldNullability() {
         List<Object> inputs = new ArrayList<>();

paimon-common/src/test/java/org/apache/paimon/predicate/TrimTransformTest.java

@@ -101,6 +101,18 @@ public void testNormalInputs() {
         assertThat(result).isEqualTo(BinaryString.fromString(" aa"));
     }
 
+    @Test
+    public void testNullCharsToTrimYieldsNull() {
+        List<Object> inputs = new ArrayList<>();
+        inputs.add(new FieldRef(0, "f0", DataTypes.STRING()));
+        inputs.add(new FieldRef(1, "f1", DataTypes.STRING()));
+        GenericRow row = GenericRow.of(BinaryString.fromString("  x  "), null);
+
+        for (TrimTransform.Flag flag : TrimTransform.Flag.values()) {
+            assertThat(new TrimTransform(inputs, flag).transform(row)).isNull();
+        }
+    }
+
     @Test
     public void testSubstringRefInputs() {
         List<Object> inputs = new ArrayList<>();