sd-bus: work around ubsan warning

ubsan complains that we add an offset to a NULL ptr here in some cases.
Which isn't really a bug though, since we only use it as the end
condition for a for loop, but we can still fix it...

Fixes: #15522

src/basic/strv.h

@@ -111,7 +111,7 @@ bool strv_overlap(char * const *a, char * const *b) _pure_;
              (s)--)
 
 #define STRV_FOREACH_PAIR(x, y, l)               \
-        for ((x) = (l), (y) = (x+1); (x) && *(x) && *(y); (x) += 2, (y) = (x + 1))
+        for ((x) = (l), (y) = (x) ? (x+1) : NULL; (x) && *(x) && *(y); (x) += 2, (y) = (x + 1))
 
 char **strv_sort(char **l);
 void strv_print(char * const *l);