Let lonely modifiers pass through if no command assigned

[lo48576]

This change prevents modifiers, "Control_L" without normal key for
example, from being eaten by libskk.

This is essentially doing same thing as 10555a2, except for returning false to let modifiers pass through.

[lo48576]

In my environment, modifiers such as Control_L causes unexpected behaviour without this patch.

Using fcitx-skk and libskk-1.0.5:

p o c Bksp →ぽ
p o c Ctrl → ぽc
p o c Ctrl Bksp Bksp →ぽc
p o c Ctrl a →ぽあ
p o c C-h Bksp a →ぽあ

• "c a" is "か" character, C-h is "delete" command in my environment.
• C-h is not working correctly, maybe because Ctrl of C-h is causing problem before the delete command is executed.
• detail in Japanese: https://mastodon.cardina1.red/@lo48576/100774943581302466.

This patch fixes behaviour in these cases (and it work as expected in my env.)

I can't reproduce this problem by using skk command, and I don't know how to write tests for this.