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

Change-Id: I2b5e39fd8a3819e07077c2a4f28a9aa5ac432e1e
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>