GetIndexerTip returns all zeros value of TipHeader instead of nil when the result is null in the JSON response

[doitian]

When the RPC GetIndexerTip returns such response

{"jsonrpc":"2.0","result":null,"id":1}

The returned *types.TipHeader is not nil, but instead a default value which block number is 0, and block hash is all zeros.

Workaround: check whether BlockHash is all zeros instead of checking whether the result is nil.