Another point:

to use a NULL or a '' are two different thing.

Variant NULL : I don't know the information and data type
Text NULL : I don't know the information, i known it is a Text only.
Text '' : i know the information, it is none information.

Then, before, you have to choice what you describe into the field,
and after you have no issues respect to the allocation space.

That is, if you use '' like a surrogate NULL,
you cannot input a none information.

Fabio