[core] Fix null partition value mismatch in PartitionsTable filterByPredicate

filterByPredicate used raw p.spec().get(key) which renders null as literal
"null", while toRow substitutes null with defaultPartitionName. This caused
predicate pushdown to fail matching null-valued partitions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sundapeng

paimon-core/src/main/java/org/apache/paimon/table/system/PartitionsTable.java

@@ -429,6 +429,7 @@ private List<Partition> filterByPredicate(
                 return partitions;
             }
             List<String> partitionKeys = fileStoreTable.partitionKeys();
+            String defaultPartitionName = fileStoreTable.coreOptions().partitionDefaultName();
             return partitions.stream()
                     .filter(
                             p -> {
@@ -437,9 +438,10 @@ private List<Partition> filterByPredicate(
                                     if (i > 0) {
                                         sb.append("/");
                                     }
+                                    String value = p.spec().get(partitionKeys.get(i));
                                     sb.append(partitionKeys.get(i))
                                             .append("=")
-                                            .append(p.spec().get(partitionKeys.get(i)));
+                                            .append(value == null ? defaultPartitionName : value);
                                 }
                                 return partitionPredicate.test(
                                         GenericRow.of(BinaryString.fromString(sb.toString())));