CLEANUP: log: lf_text_len() returns a pointer not an integer

In c83684519 ("MEDIUM: log: add the ability to include samples in logs")
we checked the return value of lf_text_len() as an integer instead of
comparing the pointer with NULL explicitly. Since this may be confusing,
let's test the return value against NULL.

[ada: for backports, the patch needs to be applied manually because of
 c6a713842 ("MINOR: log: simplify last_isspace in sess_build_logline()")]

src/log.c

@@ -3038,7 +3038,7 @@ int sess_build_logline(struct session *sess, struct stream *s, char *dst, size_t
 							  key ? key->data.u.str.data : 0,
 							  dst + maxsize - tmplog,
 							  tmp);
-				if (ret == 0)
+				if (ret == NULL)
 					goto out;
 				tmplog = ret;
 				break;