task-list-view: Set 12px top margin when list is not empty

The "new task" row doesn't stand out visually from the other list
items. Increasing the margin-top on GtdNewTaskRow a bit would
improve this by separating it from the existing rows.

This commit implement changing the top margin to 12px when there
are visible tasks.

Fixes: #105
1 job for wip/gbsneto/newtask-row-top-margin in 4 minutes and 38 seconds (queued for 1 second)
Status Job ID Name Coverage
passed #603