Return NULL and not "false" in getOracleWhereClause when the condition cannot be translated to Oracle.
This was forgotten in commit 376e5c2382dc10cae71434d98f5a478a1f300aa9 which changed the signature from "bool" to "char *".
The actual behaviour does not change because "false" is defined as 0, but it is a bug anyway and caused a warning on OS X.
Noted by Shigeru Hanada.