Fix unexpanded glob case

scripts/source_plugins.sh

@@ -16,6 +16,9 @@ silently_source_all_tmux_files() {
 	local plugin_tmux_files="$plugin_path*.tmux"
 	if plugin_dir_exists "$plugin_path"; then
 		for tmux_file in $plugin_tmux_files; do
+			# if the glob didn't find any files this will be the
+			# unexpanded glob which obviously doesn't exist
+			[ -f "$tmux_file" ] || continue
 			# runs *.tmux file as an executable
 			$tmux_file >/dev/null 2>&1
 		done