public function whereNull($column, $boolean = 'and', $not = false) {
		$null = $not ? 'not null' : 'null';
		
		return $this->on($column, 'is', $null, $boolean, false);
	}
public function whereNull($column, $boolean = 'and', $not = false) {
		$null = $not ? 'not null' : 'null';
		
		return $this->on($column, 'is', $null, $boolean, false);
	}
public function whereNull ( $ column , $ boolean = 'and' , $ not = false ) { $ null = $ not ? 'not null' : 'null' ; return $ this -> on ( $ column , 'is' , $ null , $ boolean , false ) ; }
Add an "on where is null" clause to the join.

@param string $column
@param string $boolean
@return \queryBuilder\JoinClause
Add an on where is null clause to the join .