I think you could make it slightly tidier, but the definition here seems to me to be the right way to go... but you need to decide whether you want to fail if a variable is empty, or only if it's not defined at all.  The previous example shows how to check if it's defined at all; this one assumes an empty variable is still an error:

exists = $(if $($1),$($1),$(error $1 has no value))

foo: @echo $(call exists,FOO)

It would be nicer if there were some syntactic sugar for calls to make this shorter.  Also, one disadvantage of this is that the variable is evaluated twice, which is a drag... the other option (checking for undefined) is more efficient in that respect.

GNU Make currently has very little inherent support for debugging makefiles.  One simple change could help a lot:  adding a new type of variable reference that requires that the variable be defined.  If the variable were not defined, make would stop with an informative error message.  For example, if the syntax of a required reference were $(!varname), then one could write:

%.a:
    ar rs $@ $(!^)

This would mean that it would be required that $^ be non-empty so that this rule would not create an empty archive file.

I've come across dozens of situations where I would use this feature if it existed.  On many occasions I've spent half an hour figuring out why a makefile doesn't work, when this feature would have told me instantly what the problem was.