when expansion value is "", the original macro isn't reinserted on quit

[nverno]

This isn't a problem in elisp since empty macros are replaced with nil, but I just made an extension for makefiles, https://github.com/nverno/macrostep-make, and noticed that when a macro's
definition hasn't been defined, eg.
CC =
then, when
${CC}
is expanded, it is blank but doesn't replace with ${CC} upon exit from macroexpansion.
A workaround is to expand with " " instead of "". I don't know if this is something to fix on
this end, but thought I would mention it.

THanks