Nicer handleing of NULL in execute with format.

When using executeUpdateWithFormat encountered following issue,
%@ replaced by @ for NULL objects and would fail with non descriptive lastErrorMessage.

example 

    [db executeUpdateWithFormat@"INSERT INTO TEST VALUES(%@)", NULL];
    // results in cleanedSQL == "INSERT INTO TEST VALUES(@)"
    // fix changes this to cleanedSQL == "INSERT INTO TEST VALUES(NULL)"