* idl_gen_rust.cpp: Fixgoogle/flatbuffers#5849 (Option/required-aware
codegen for unions)
The generated code was assuming that an Option is always returned by the
union-getter method: however, this is only true if the field is not
marked as `(required)`.
* idl_gen_rust.cpp: flip conditional
* idl_gen_rust.cpp: Add comment, as requested in review
The code added is not covered by the integration test
