minimize_wdba: fix handling of input with useless SCCs
* spot/twaalgos/minimize.cc (minimize_wdba): Diminish the color of terminal SCCs that are incomplete, as if they had a non-accepting sink as successor. * spot/twaalgos/strength.hh, spot/twaalgos/strength.cc (is_terminal_automaton): Add an option to ignore trivial SCC as we did before, since it matters for deciding membership to the guarantee class. (is_safety_mwdba): Rewrite as ... (is_safety_automaton): ... generalizating to any acceptance, and ignoring trivial SCCs. * bin/ltlfilt.cc, python/ajax/spotcgi.in, spot/tl/hierarchy.cc, tests/core/ikwiad.cc: Adjust usage of is_terminal_automaton and is_safety_automaton(). * tests/core/hierarchy.test: Add a problematic formula as test-case. * NEWS: Mention the bug.