build: fix test-nss.c build failure with --disable-{resolved,myhostname} (#3081)

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).

src/test/test-nss.c

@@ -381,12 +381,18 @@ static void test_byaddr(void *handle,
 
 #ifdef HAVE_MYHOSTNAME
 #  define MODULE1 "myhostname\0"
+#else
+#  define MODULE1
 #endif
 #ifdef HAVE_RESOLVED
 #  define MODULE2 "resolve\0"
+#else
+#  define MODULE2
 #endif
 #ifdef HAVE_MACHINED
 #  define MODULE3 "mymachines\0"
+#else
+#  define MODULE3
 #endif
 #define MODULE4 "dns\0"