The only thing that does not have a form is an abstract object like numbers (e.g. the number 3 in essence does not have a form) or concepts like love (e.g. love is a feeling, what form does it have?) and philosophers have actually disputed whether we can say that abstract objects like numbers do exist. There is no entity called number 7 that exists independently of anything, for the number 7 by necessity is an expression of the quantity of things.
You can't call Allah an abstract object, because abstract objects don't have causal effects (e.g. the number 7 can't cause something to happen, but we all know that Allah caused the universe to come into existence, hence He is not an abstract object).
You can't call Allah a mere concept, for He has an actual Self who speaks with us. Concepts don't have centers of self consciousness, but Allah does.
Christian philosophers (and I am assuming Ashari philosophers) will argue back that Allah is like an unembodied mind (I don't have serious objections to the meaning of this statement as I do with the actual words used), thus arguing that Allah does not have a form. However, just because Allah does not have a body, that does not necessitate that He doesn't have a form. Even Casper the friendly ghost in the cartoons has a form, yet no body. So if our limited minds could imagine this for the creation, then what about the creator?
If Allah does not have a form then what are we going to look at on the Day of Judgment?
The Prophet (peace be upon him) said we will see Allah as clearly as the moon on a cloudless night. How on earth does that happen if Allah has nothing of Himself for us to see?
Just as bro Hanbali said, if you say that He does not have a form you are basically nullifying His existence