Print error if Shenandoah flags are empty (instead of crashing)

Providing '-XX:ShenandoahGCMode=' or '-XX:ShenandoahGCHeuristics=' (when parsed) causes a crash without meaningful info. E.g.:

java -XX:+UseShenandoahGC -XX:ShenandoahGCMode= -version
java -XX:+UseShenandoahGC -XX:ShenandoahGCMode=satb -XX:ShenandoahGCHeuristics= -version
java -XX:+UseShenandoahGC -XX:+UnlockExperimentalVMOptions -XX:ShenandoahGCMode=iu -XX:ShenandoahGCHeuristics= -version
java -XX:+UseShenandoahGC -XX:+UnlockDiagnosticVMOptions -XX:ShenandoahGCMode=passive -XX:ShenandoahGCHeuristics= -version

Instead it can be reported similar to '=wrong', e.g:

Error occurred during initialization of VM
Unknown -XX:ShenandoahGCMode option (null)