tree-wide: make invocations of extract_first_word more uniform (#4627)

extract_first_words deals fine with the string being NULL, so drop the upfront
check for that.

src/shared/condition.c

@@ -111,9 +111,8 @@ static int condition_test_kernel_command_line(Condition *c) {
                 return r;
 
         equal = !!strchr(c->parameter, '=');
-        p = line;
 
-        for (;;) {
+        for (p = line;;) {
                 _cleanup_free_ char *word = NULL;
                 bool found;