glib-aux: add NM_STRV_EMPTY() helper

NM_PTRARRAY_EMPTY(char*) gives a "char *const*" pointer,
which is often not useful where a strv array is needed.
Add a helper macro.

src/libnm-glib-aux/nm-shared-utils.h

@@ -522,6 +522,7 @@ gboolean nm_utils_memeqzero(gconstpointer data, gsize length);
 extern const void *const _NM_PTRARRAY_EMPTY[1];
 
 #define NM_PTRARRAY_EMPTY(type) ((type const *) _NM_PTRARRAY_EMPTY)
+#define NM_STRV_EMPTY()         ((char **) _NM_PTRARRAY_EMPTY)
 
 static inline void
 _nm_utils_strbuf_init(char *buf, gsize len, char **p_buf_ptr, gsize *p_buf_len)