Function isNull

isNull(expression): OGCCondition
Creates an "is null" expression. It evaluates to true if the value of the expression is null.
```
  FilterFactory.isNull( FilterFactory.property( "city_name" ) );

  FilterFactory.isNull( FilterFactory.func( "getCityName" ) );
```
Parameters
- expression: Expression
  the expression to evaluate
Returns OGCCondition
the isNull expression