Add braces to empty conditional blocks

Add braces to empty conditional blocks

GCC 7+ is able to quickly optimize away empty if/else blocks if the braces are
provided.  While this adds some additional syntax, it should also result
in faster optimization, so change our empty blocks after conditionals to use
braces.

FIXES 134377

Reviewed-on: https://gerrit.openafs.org/13081
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit a411366f57dcf39cc17b6d61d8332e520dff57d1)

Change-Id: I9243105d75878487ec965070d3946e8d683f264b
Reviewed-on: https://gerrit.openafs.org/13149
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: PatRiehecky <jcpunk@gmail.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>