sd-json: make sure SD_JSON_BUILD_STRING_UNDERSCORIFY() can deal with NULL strings

SD_JSON_BUILD_STRING() and everything else can deal with it, make sure
SD_JSON_BUILD_STRING_UNDERSCORIFY() can too.

(cherry picked from commit a77a95665f)
(cherry picked from commit 29f4fba3c6)
(cherry picked from commit a09efa6cf5)

src/libsystemd/sd-json/sd-json.c

@@ -3532,7 +3532,7 @@ _public_ int sd_json_buildv(sd_json_variant **ret, va_list ap) {
                         if (current->n_suppress == 0) {
                                 _cleanup_free_ char *c = NULL;
 
-                                if (command == _JSON_BUILD_STRING_UNDERSCORIFY) {
+                                if (command == _JSON_BUILD_STRING_UNDERSCORIFY && p) {
                                         c = strreplace(p, "-", "_");
                                         if (!c) {
                                                 r = -ENOMEM;