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 #21 from Mind4SE/fix-evaluator

If the option isn't found (=null), return false instead of NPE.