Except for @Martin Hansen's A (where all four versions return nothing but
#N/A for me) two of the above As do distinguish:
a "blank" cell (truly blank OR containing a formula returning `""`)
a "non-blank" cell (whether Text, Numeric, Boolean or other Formula).
They are all however much longer than they need to be if just for the purpose of differentiation. Much of the length is due to the text chosen to notify the condition. Such text is not necessary (I would like it to return true...). A correct A to Is the cell "not-blank" would serve for differentiation (as @CamSyl's interpretation).
However in that case a correct A to Is the cell "blank" would serve equally well (or better, given OP wants
true for blank), and that can be achieved with a formula roughly a third of the length of @CamSyl's solution (which does not solve OP's problem anyway).
I recommend a formula of the type:
(which is all the essence of @Mehper C. Palavuzlar's A without the text).