user_output_names_find_next: Always initialize target_output

This way, if the user has provided a valid, existing output in the list
of outputs, the focus & move workspace to output commands will not
report a misleading failure.

Side-effect is that the command code will try to execute a no-op e.g. by
moving the workspace to the output it already is on. But that's what the
user is actually requesting in this case and it shouldn't be a problem.

Fixes #4691

testcases/t/543-move-workspace-to-multiple-outputs.t

@@ -39,6 +39,16 @@ sub is_ws {
     is(get_output_for_workspace("$ws_num"), "fake-$out_num", "Workspace $ws_num -> $out_num: $msg");
 }
 
+###############################################################################
+# Test moving workspace to same output
+# See issue #4691
+###############################################################################
+is_ws(1, 0, 'sanity check');
+
+my $reply = cmd '[con_mark=aa] move workspace to output fake-0';
+is_ws(1, 0, 'workspace did not move');
+ok($reply->[0]->{success}, 'reply success');
+
 ###############################################################################
 # Test using "next" special keyword
 ###############################################################################