[NVIDIA TF] Throw error only for non-empty function definitions.
@reedwm what do you think ? Thanks for the PR! Quick question, 
>  there can be Node ops with type func with default values that have no associated name.

When does this happen? @k-w-w we tried to register an OP that can define or not an optional `func`. I see, can you add this as a comment in the code? (`# Custom ops may contain a func attr with an empty fname`) 
During Function Serialization and Deserialization with Saved Models there can be Node ops with type `func` with default values that have no associated name. In such cases, we shouldn't look for undefined empty strings in Function Library. This is a trivial change and helps with how Saved Models are loaded. 