Go: add support for empty tags-set when loading saved model
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There is a check in `saved_model.go` that does not allow empy tag set, however there are models without tags that work perfectly in the library without providing them (they are optional in the Python library too, for example).

Apparently the check was added in #36466, though instead of throwing an error on an empty set, a nil value can be passed in these cases and the C library will not complain. Otherwise, the pointer to the first element is passed as before.