If the option isn't found (=null), return false instead of NPE.
This fix is very useful for our mindoc "plugin-ification" when using
static command-line options, with other plugin-contributed flags loading
disabled, and encountering such flags lead us to this "null" value as
flag wasn't found (as its provider wasn't loaded).
Merge pull request #14 from Mind4SE/fix-evaluator

Fix evaluator.
If the option isn't found (=null), return false instead of NPE.
Merge pull request #14 from MIND-Tools/master

Re-sync with MIND-Tools to integrate MIND-Tools PR#14 (reviewed fix-evaluator)