validate if cached value is empty before use (#13830)

fixes a crash reproduced while running hadoop tests

```
goroutine 201564 [running]:
github.com/minio/minio/cmd.metaCacheEntries.resolve({0xc0206ab7a0, 0x4, 0xc0015b1908}, 0xc0212a7040)
	github.com/minio/minio/cmd/metacache-entries.go:352 +0x58a
```

Bonus: HeadBucket() should always provide content-type

cmd/bucket-handlers.go

@@ -1210,7 +1210,7 @@ func (api objectAPIHandlers) HeadBucketHandler(w http.ResponseWriter, r *http.Re
 		return
 	}
 
-	writeSuccessResponseHeadersOnly(w)
+	writeResponse(w, http.StatusOK, nil, mimeXML)
 }
 
 // DeleteBucketHandler - Delete bucket

cmd/erasure-server-pool.go

@@ -1131,7 +1131,9 @@ func (z *erasureServerPools) ListObjects(ctx context.Context, bucket, prefix, ma
 	}
 	merged, err := z.listPath(ctx, &opts)
 	if err != nil && err != io.EOF {
-		logger.LogIf(ctx, err)
+		if !isErrBucketNotFound(err) {
+			logger.LogIf(ctx, err)
+		}
 		return loi, err
 	}
 

cmd/metacache-entries.go

@@ -307,7 +307,7 @@ func (m metaCacheEntries) resolve(r *metadataResolutionParams) (selected *metaCa
 		// shallow decode.
 		xl, err := entry.xlmeta()
 		if err != nil {
-			logger.LogIf(context.Background(), err)
+			logger.LogIf(GlobalContext, err)
 			continue
 		}
 		objsValid++
@@ -339,11 +339,17 @@ func (m metaCacheEntries) resolve(r *metadataResolutionParams) (selected *metaCa
 	if objsValid < r.objQuorum {
 		return nil, false
 	}
+
 	// If all objects agree.
 	if selected != nil && objsAgree == objsValid {
 		return selected, true
 	}
 
+	// If cached is nil we shall skip the entry.
+	if selected.cached == nil {
+		return nil, false
+	}
+
 	// Merge if we have disagreement.
 	// Create a new merged result.
 	selected = &metaCacheEntry{