Before this commit, if the value didn't begin with a dollar sign, _zsh_highlight_main_highlighter__try_expand_parameter() would return 1 by accident.¹ Tweak the input validation to make this behaviour explicit. No functional change. ¹ Specifically, it would return 1 because ${parameter_name}'s value would be the empty string and ${parameter_name_pattern} wouldn't match that.pull/718/head