Drop return typehint on QuerySpecification

[Brammm]

Addresses #12

[frankdejonge]

@Brammm looks good! Can you add a testcase that validates support?

[Brammm]

@frankdejonge I just noticed how you circumvented the whole "isNullreturns string" thing in the FieldIsNotNull test:

return $expr->andX($expr->isNotNull("{$rootAlias}.{$this->field}"));

So I could either change that test to return the string (and make sure everything still works) or drop this PR and do the same in my project...

[frankdejonge]

@Brammm both work for me :)