Re-implementing UnaryExpression to make it more simple
Allowing isNull and isNotNull to accept ExpressionInterface
Automatically quoting strings passed to isNull and isNotNull

All other methods in QueryExpression had the same behavior so it
made sense to support that here too. Fixes #3836
Fix docblock errors.
Merge pull request #3844 from cakephp/3.0-fix-notnull

3.0 fix notnull missing identifier quoting