refs.c: allow passing NULL to ref_transaction_free

refs.c: allow passing NULL to ref_transaction_free

Allow ref_transaction_free(NULL) as a no-op. This makes ref_transaction_free
easier to use and more similar to plain 'free'.

In particular, it lets us rollback unconditionally as part of cleanup code
after setting 'transaction = NULL' if a transaction has been committed or
rolled back already.

Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Ronnie Sahlberg <redacted>
Signed-off-by: Junio C Hamano <redacted>
Acked-by: Michael Haggerty <redacted>