Fix marshal function with nullable fragments

[vkondrav]

While upgrading to release 2.1.0 I came across a strange problem where the generated marshal function was no longer checking nullability of fragment before executing writer.writeFragment. This resulted in a null pointer exception when calling marshaller(). Although this did not cause any problem during normal execution we do have a point in the app where we write a fragment back to apolloStore. This is where the NullPointer Exception would occur.

Before

    public ResponseFieldMarshaller marshaller() {
      return new ResponseFieldMarshaller() {
        @Override
        public void marshal(ResponseWriter writer) {
          writer.writeFragment(downloadAppDeepLinkFragment.marshaller());
        }
      };
    }

After

    public ResponseFieldMarshaller marshaller() {
      return new ResponseFieldMarshaller() {
        @Override
        public void marshal(ResponseWriter writer) {
          final DownloadAppDeepLinkFragment $downloadAppDeepLinkFragment = downloadAppDeepLinkFragment;
          if ($downloadAppDeepLinkFragment != null) {
            $downloadAppDeepLinkFragment.marshaller().marshal(writer);
          }
        }
      };
    }

[apollo-cla]

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[sav007]

Thx !!!