Harshavardhana b120bcb60a
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
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