[CLIENT] fixed NPE in GenericJsonRecord (apache#10482)

Fixes apache#10472

GenericJsonRecord should not error out if the non-existent field is queried.

Added a null check and returning null if the field does not exist.

Extended one of the tests for GenericJsonRecord.

(cherry picked from commit ff076e3)

pulsar-client-api/src/main/java/org/apache/pulsar/client/api/schema/GenericRecord.java

@@ -58,7 +58,7 @@ default Object getField(Field field) {
      * Retrieve the value of the provided <tt>fieldName</tt>.
      *
      * @param fieldName the field name
-     * @return the value object
+     * @return the value object, or null if field doesn't exist
      */
     Object getField(String fieldName);
 

pulsar-client/src/main/java/org/apache/pulsar/client/impl/schema/generic/GenericJsonRecord.java

@@ -61,6 +61,9 @@ public JsonNode getJsonNode() {
     @Override
     public Object getField(String fieldName) {
         JsonNode fn = jn.get(fieldName);
+        if (fn == null) {
+            return null;
+        }
         if (fn.isContainerNode()) {
             AtomicInteger idx = new AtomicInteger(0);
             List<Field> fields = Lists.newArrayList(fn.fieldNames())

pulsar-client/src/test/java/org/apache/pulsar/client/impl/schema/JSONSchemaTest.java

@@ -45,9 +45,12 @@
 import org.apache.pulsar.client.impl.schema.SchemaTestUtils.Foo;
 import org.apache.pulsar.client.impl.schema.SchemaTestUtils.NestedBar;
 import org.apache.pulsar.client.impl.schema.SchemaTestUtils.NestedBarList;
+import org.apache.pulsar.client.impl.schema.generic.GenericJsonRecord;
 import org.apache.pulsar.client.impl.schema.generic.GenericSchemaImpl;
 import org.apache.pulsar.common.schema.SchemaInfo;
 import org.apache.pulsar.common.schema.SchemaType;
+import org.assertj.core.api.Assertions;
+import org.json.JSONException;
 import org.skyscreamer.jsonassert.JSONAssert;
 import org.testng.Assert;
 import org.testng.annotations.Test;
@@ -451,5 +454,8 @@ public void testJsonGenericRecordBuilder() {
         assertEquals("USA", seller3Record.getField("state"));
         assertEquals("oakstreet", seller3Record.getField("street"));
         assertEquals(9999, seller3Record.getField("zipCode"));
+
+        assertTrue(pc3Record instanceof GenericJsonRecord);
+        Assertions.assertThatCode(() -> pc3Record.getField("I_DO_NOT_EXIST")).doesNotThrowAnyException();
     }
 }