nss-systemd,strv: use a common definition of a non-NULL but empty strv

Follow-up for: 239903d44c

src/basic/strv.h

@@ -221,3 +221,9 @@ int string_strv_ordered_hashmap_put(OrderedHashmap **h, const char *key, const c
 int strv_rebreak_lines(char **l, size_t width, char ***ret);
 
 char** strv_filter_prefix(char * const *l, const char *prefix);
+
+/* whenever we need to initialize something with a constant non-NULL, but empty strv, we can use this shared
+ * one */
+extern const char* const strv_empty[1];
+
+#define STRV_EMPTY ((char**) strv_empty)