Router: match when pattern and tested string are both zero length

Otherwise, undefined behaviour will be triggered.

Can be reproduced by test/test_routing.py::test_routes_match_host_empty
with enabled UndefinedBehaviorSanitizer:

src/nxt_http_route.c:2141:17: runtime error: applying zero offset to null pointer
    #0 0x100562588 in nxt_http_route_test_rule nxt_http_route.c:2091
    nginx#1 0x100564ed8 in nxt_http_route_handler nxt_http_route.c:1574
    nginx#2 0x10055188c in nxt_http_request_action nxt_http_request.c:570
    nginx#3 0x10052b1a0 in nxt_h1p_request_body_read nxt_h1proto.c:998
    nginx#4 0x100449c38 in nxt_event_engine_start nxt_event_engine.c:542
    nginx#5 0x100436828 in nxt_thread_trampoline nxt_thread.c:126
    nginx#6 0x18133e030 in _pthread_start+0x84 (libsystem_pthread.dylib:arm64e+0x7030)
    nginx#7 0x181338e38 in thread_start+0x4 (libsystem_pthread.dylib:arm64e+0x1e38)

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior src/nxt_http_route.c:2141:17

src/nxt_http_route.c

@@ -2134,6 +2134,10 @@ nxt_http_route_pattern(nxt_http_request_t *r, nxt_http_route_pattern_t *pattern,
         return 0;
     }
 
+    if (nxt_slow_path(length == 0)) {
+        return 1;
+    }
+
     nxt_assert(pattern->u.pattern_slices != NULL);
 
     pattern_slices = pattern->u.pattern_slices;