This patch fixes cases where a transaction caused empty writeset to be
replicated. This could happen in the case where a transaction executes
a statement that initially manages to modify some data and therefore
appended keys some for  certification. The statement is however rolled
back at some later stage due to some error (for example, a duplicate
key error). After statement rollback the transaction is still alive,
has no other changes. When committing such transaction, an empty
writeset was replicated through Galera.

The fix is to avoid calling into commit hook only when transaction
has appended one or keys for certification *and* has some data in
binlog cache to replicate. Otherwise, the commit is considered empty,
and goes through usual empty commit path.
Signed-off-by: Julius Goryavsky <julius.goryavsky@mariadb.com>