Always return a valid string on urid-unmap, special fix for Ingen

Ingen is calling urid-unmap with URID 62465.
There's no way that many URIs have been mapped, so something is weird in Ingen.
Ingen aborts when it receives NULL on urid-unmap (caused by a bad URID),
so as a workaround we'll return a valid but somewhat null URI.
Ingen still needs fixing though.

source/bridges-ui/CarlaBridgeUI-LV2.cpp

@@ -654,11 +654,12 @@ public:
 
     const char* getCustomURIDString(const LV2_URID urid) const noexcept
     {
-        CARLA_SAFE_ASSERT_RETURN(urid != CARLA_URI_MAP_ID_NULL, nullptr);
-        CARLA_SAFE_ASSERT_RETURN(urid < fCustomURIDs.count(), nullptr);
+        static const char* const sFallback = "urn:null";
+        CARLA_SAFE_ASSERT_RETURN(urid != CARLA_URI_MAP_ID_NULL, sFallback);
+        CARLA_SAFE_ASSERT_RETURN(urid < fCustomURIDs.count(), sFallback);
         carla_debug("CarlaLv2Client::getCustomURIDString(%i)", urid);
 
-        return fCustomURIDs.getAt(urid, nullptr);
+        return fCustomURIDs.getAt(urid, sFallback);
     }
 
     // ---------------------------------------------------------------------