loop_start hook isn't always called (workflow bug)

[Denis-de-Bernardy]

while investigating a weird bug in a plugin, I noticed that, if you do a search that returns no results, the loop_start hook never gets called -- but the loop_end hook does.

imo, loop_end should not get called if loop_start wasn't.

[markjaquith]

(In [8188]) Don't call loop_end hook if we don't have any posts. props Denis-de-Bernardy and DD32. fixes #6769 for trunk