I can't help but speculate that rather than being an ealar Ungoliant seems more like a kind of 'manifestation' of the 'Void'. She comes across as an 'absence' rather than a 'presence' in so many ways. She swallows, consumes, the light.
This is pure speculation, but I wonder, as it seems the 'Void' was a 'place' where Melkor could wander before the creation of Arda, whether something of that 'emptiness' entered into the Music through him, that it was some part of the theme that he introduced - the place where he sought the Secret Fire but failed to find it. She exists because Melkor sang her into being...
Of course, this drags into the realm of metaphysics in a big way
This could account for Ungoliant's 'existence'...or it may just be a mad idea
