When building without resolved and/or myhostname, test-nss.c failed to build
with
src/test/test-nss.c: In function 'main':
src/test/test-nss.c:417:32: error: 'MODULE1' undeclared (first use in this function)
NULSTR_FOREACH(module, MODULE1 MODULE2 MODULE3 MODULE4) {
^
Ensure that all MODULEx are always defined, and empty if the module is not
available (so that it will be a no-op in the string concatenation).