[mlir][tblgen] Fix crash in `-gen-enum-(decls|defs)` when `EnumAttrInfo` is not defined

This should instead simply produce an empty file instead of crashing and causing a build to fail. This allows one to be more lazy in cmake code and just always generate the files for enum attributes, even if none are ever defined in the dialect.

Differential Revision: https://reviews.llvm.org/D135355