fix(shmem): don't cache nullptr in GetRegionFromCache

A failed region lookup was inserted into the thread-local cache as
nullptr, making the failure permanent for the lifetime of the cache
generation - retrying never healed because the fast path would return
nullptr without calling GetRegion again. Skip the cache insert on
failure so subsequent calls retry the slow path.

fairmq/shmem/Manager.h

@@ -390,8 +390,10 @@ class Manager
         }
 
         auto* lRegion = GetRegion(id);
+        if (lRegion) {
             fTlRegionCache.fRegionsTLCache.emplace_back(lRegion, id, fShmId64);
             fTlRegionCache.fRegionsTLCacheGen = fRegionsGen;
+        }
         return lRegion;
     }