Fix NPE when emitting null BigQuery record fields.

When a record field on a class were marshalling is null, we used to throw an exception.

java.lang.NullPointerException
  at com.google.appengine.tools.mapreduce.impl.BigQueryDataMarshallerByType.mapFieldNameToValue(BigQueryDataMarshallerByType.java:54)
  at com.google.appengine.tools.mapreduce.impl.BigQueryDataMarshallerByType.mapFieldNameToValue(BigQueryDataMarshallerByType.java:72)
  at com.google.appengine.tools.mapreduce.impl.BigQueryMarshallerByType.toBytes(BigQueryMarshallerByType.java:61)
  at com.google.appengine.tools.mapreduce.BigQueryDataMarshallerTest.toJsonString(BigQueryDataMarshallerTest.java:81)
  at com.google.appengine.tools.mapreduce.BigQueryDataMarshallerTest.testGeneratedJson(BigQueryDataMarshallerTest.java:60)
  at com.google.appengine.tools.mapreduce.BigQueryDataMarshallerTest.testGeneratedJsonForClassWithRecord(BigQueryDataMarshallerTest.java:347)

With this fix we'll now emit 'null' as the json value. For example:  {'record':null}.

java/src/main/java/com/google/appengine/tools/mapreduce/impl/BigQueryDataMarshallerByType.java

@@ -51,6 +51,9 @@ public BigQueryDataMarshallerByType(Map<Field, BigqueryFieldMarshaller> marshall
    * @return a nested map of field name to field value.
    */
   Map<String, Object> mapFieldNameToValue(Object toMap) {
+    if (toMap == null) {
+      return null;
+    }
     Class<?> typeOfObjectToMap = toMap.getClass();
     Map<String, Object> toRet = new HashMap<>();
     Set<Field> fieldsToMap = getFieldsToSerialize(typeOfObjectToMap);

java/src/test/java/com/google/appengine/tools/mapreduce/BigQueryDataMarshallerTest.java

@@ -73,7 +73,7 @@ public void testGeneratedJson(String expected, T actual) {
       try {
         assertTrue(mapper.readTree(expected).equals(mapper.readTree(actualJson)));
       } catch (IOException e) {
-        fail("Exception while serializing. Expected " + expected);
+        fail("Exception while serializing. Expected " + expected + " got " + actualJson);
       }
     }
 
@@ -338,6 +338,44 @@ public ClassWithMap(Map map, int id) {
     }
   }
 
+  @SuppressWarnings("unused")
+  public void testGeneratedJsonForClassWithRecord() {
+    BigQueryDataMarshallerTester<ClassWithRecord> tester =
+      new BigQueryDataMarshallerTester<ClassWithRecord>(
+        new BigQueryMarshallerByType<ClassWithRecord>(ClassWithRecord.class));
+
+    tester.testGeneratedJson("{\"record\":null}",
+      new ClassWithRecord(null));
+
+    tester.testGeneratedJson("{\"record\":{\"name\":\"myname\",\"value\":\"myvalue\"}}",
+      new ClassWithRecord(new ClassWithRecord.Record("myname","myvalue")));
+
+    tester.testSchema(new TableSchema().setFields(Lists.newArrayList(new TableFieldSchema()
+      .setName("record").setType("record").setFields(Lists.newArrayList(new TableFieldSchema()
+        .setName("name").setType("string"), new TableFieldSchema()
+        .setName("value").setType("string"))))));
+  }
+
+
+  private static class ClassWithRecord {
+    private static class Record{
+      public String name;
+      public String value;
+      public Record(String name, String value){
+          this.name=name;
+          this.value=value;
+      }
+    }
+
+    @SuppressWarnings("unused")
+    Record record;
+
+    @SuppressWarnings("unused")
+    public ClassWithRecord(Record record) {
+      this.record = record;
+    }
+  }
+
   public void testGeneratedJsonForClassExtendingAbstractClass() {
     BigQueryDataMarshallerTester<ClassExtendingAbstract> tester =
         new BigQueryDataMarshallerTester<>(