Fix a crash in reports using the mysql storage backend.

This crash was caused by the fact that the report, after requesting
the transaction list, did not check that the transactions are inside
the requested date interval. Since the date is used to compute an
index in a columns list a date outside the requested interval would
cause an invalid index to be computed and causing the crash.

Now the report performs a validity check of the date after filtering
the transactions.

Also fixed the SQL backend implementation to properly filter by dates
even if an empty split filter is provided. Also added a testcase for
this scenario.