chore: Check if projectId is already set from the MDS call (#1904)

Author: lqiu96

google-auth-library-java/oauth2_http/java/com/google/auth/oauth2/ComputeEngineCredentials.java

@@ -364,9 +364,14 @@ public String getProjectId() {
 
     String projectIdFromMetadata = getProjectIdFromMetadata();
     synchronized (this) {
-      this.projectId = projectIdFromMetadata;
+      // Check first if another thread set the Project ID. No need to overwrite
+      // if a Projects ID already exists. Tries to prevent a case where the last call
+      // for `getProjectIdFromMetadata()` returns null and overwrites valid data.
+      if (this.projectId == null) {
+        this.projectId = projectIdFromMetadata;
+      }
     }
-    return projectIdFromMetadata;
+    return this.projectId;
   }
 
   private String getProjectIdFromMetadata() {