jni_util: Make JStringRef and JArrayRef null-safe.
jni_util: Make JStringRef and JArrayRef null-safe.

They return empty string and empty array (to avoid crashes) but also
report an error with backtrace to stderr when called in that manner.

wpilibsuite/ntcore,wpilibsuite/ntcore,wpilibsuite/ntcore,robotdotnet/ntcore,robotdotnet/ntcore,333fred/ntcore,333fred/ntcore,wpilibsuite/ntcore,333fred/ntcore,333fred/ntcore,robotdotnet/ntcore,robotdotnet/ntcore