message: Add null check for key in Signaturede in InterpreterBuilder.
  We don't need to fail if key is not set. method_name is the main part used which corresponds to the exported method name for the SignatureDef
diff: @@ -488,8 +488,10 @@ TfLiteStatus InterpreterBuilder::ParseSignatureDefs(
  signature_def.inputs = GetMapFromTensorMap(fb_signature_def->inputs());
  signature_def.outputs = GetMapFromTensorMap(fb_signature_def->outputs());
  signature_def.method_name = fb_signature_def->method_name()->c_str();
  + if (fb_signature_def->key() != nullptr) {
  signature_def.signature_def_key = fb_signature_def->key()->c_str();
  }
  + }
  interpreter->SetSignatureDef(std::move(signature_defs));
  return kTfLiteOk;
  }
  