Ignore $SYSTEMD_INVOKED_AS if empty

Let's treat unset SYSTEMD_INVOKED_AS and SYSTEMD_INVOKED_AS='' the same.
I encountered this when testing various --help outputs. It is often
more convenient to override a variable with VAR= instead of removing it,
in particular on the commandline. But we'd then print help with the
program name missing. We look at the variable in two places, so adjust
both as in $subject.

src/basic/argv-util.c

@@ -28,7 +28,7 @@ void save_argc_argv(int argc, char **argv) {
 }
 
 bool invoked_as(char *argv[], const char *token) {
-        const char *progname = secure_getenv("SYSTEMD_INVOKED_AS");
+        const char *progname = empty_to_null(secure_getenv("SYSTEMD_INVOKED_AS"));
 
         if (!progname && argv)
                 progname = argv[0];