Make `weechat-pcompletions-at-point' return nil when not applicable.

Fixes a bug uncovered via IRC which was never triggered on our setups.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>
Add test for bug fixed in 8e13991.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>