criteria: fix crash when comparing NULL properties

For each following combinations of criteria & command below, the command would
crash sway without the fix.
It's particular about the __focused__ criteria, where the view matches part of
the criteria but not the focused app, leading to a failure when calling
`strcmp` with NULL.

"xterm" is a non-wayland app (X11) and "kitty" is. Both are terminals.

    # "class" is specific to X11
    # The view is X11 (xterm) leading to the criteria checking for the
    # focused app's class, leading to a crash
    for_window [class="__focused__"] floating enable
    exec kitty -e xterm

    # Similarly, crash as the focused app (xterm) has no app_id when the view has one
    for_window [app_id="__focused__"] floating enable
    exec xterm -e kitty

    # If the view has a title but not the focused app: NULL title will crash criteria checking
    for_window [title="__focused__"] floating enable
    exec xterm -title "" -e xterm

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