code: private static <V> boolean isInitialized(Metadata metadata, V value) {
          if (metadata.valueType.getJavaType() == WireFormat.JavaType.MESSAGE) {
              return value != null;
          }
          return true;
      }
docs: Checks if is initialized.
  
  @param <V> the value type
  @param metadata the metadata
  @param value the value
  @return true, if is initialized