Fix topic rule for empty topic

When a topic is empty, topic regular expression never match even if
regular expression is '.*'.

Null topic are now considered a empty topic string.
Change manual trigger button to floating button

This patch changes "Trigger Selected" button to floating button.

Fix for JENKINS-21407

Task-Url: https://issues.jenkins-ci.org/browse/JENKINS-21407
Merge pull request #179 from criteo/fix-empty-topic

Fix topic rule for empty topic
Merge pull request #180 from rinrinne/floating-trigger

Change manual trigger button to floating button