rename synth.sys_fts2spec -> fts2spec
handle case of no atomic propositions
extract _conj_action from synth functions, filter '' action

This filtering allows for a "nop" action (empty letter \epsilon), to enable adding an unguarded labeled transition. Useful to avoid creation of unnecessary atomic propositions for actions.
extract create_states from synth functions
bugfix: use bool gr1c variable for 1 or 2 TS states