SmrSession: fix var uncompress error
SmrSession: fix var uncompress error

Executing `gzuncompress` on an empty session var will result in a
"data error". We can avoid this problem by checking first to see
if the session var is the empty string.

This allows us to remove the PHP silence operators from the
processing of `SmrSession::$var`.

smrealms/smr,smrealms/smr,hemberger/smr,smrealms/smr,hemberger/smr,hemberger/smr,hemberger/smr