QSqlQuery::isNull string overload

Introduce isNull overload to take field name as a parameter.

This is corresponding to the commit
7e6e1412348fb8d8df844d821ee80d6d3de69517

Change-Id: I122f79707d26eaa09c2f38dc31aeee1dac7de33b
Reviewed-by: Mark Brand <mabrand@mabrand.nl>
add mising tag to method documentation

Change-Id: Iafd7a268bf3b3be9a06ab1badc23e3db52df9586
Reviewed-by: Mark Brand <mabrand@mabrand.nl>