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/basic/extract-word.c

@@ -48,7 +48,7 @@ int extract_first_word(const char **p, char **ret, const char *separators, Extra
 
         /* Bail early if called after last value or with no input */
         if (!*p)
-                goto finish_force_terminate;
+                goto finish;
         c = **p;
 
         if (!separators)