glib-aux: fix nm_ref_string_equal_str()

Fix comparison with a NULL string.

The only current caller is nm-supplicant-interface.c, and this bug
causes a missing cleanup of the CurrentBSS property on disconnect.

Fixes: ac8c3a7111 ('glib-aux: improve nm_ref_string_equals_str() to work for non-C-strings')

https://bugzilla.redhat.com/show_bug.cgi?id=1983735
https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1073
(cherry picked from commit b755539aa6)
(cherry picked from commit 4685651e76)

src/libnm-glib-aux/nm-ref-string.h

@@ -118,7 +118,7 @@ static inline gboolean
 nm_ref_string_equal_str(NMRefString *rstr, const char *str)
 {
     if (!str)
-        return (!!rstr);
+        return !rstr;
 
     if (!rstr)
         return FALSE;