core/types: make "miner" optional in Header JSON (ethereum#24666)

"miner" is not set for pending block responses in some cases.

Fixes ethereum#24632

Author: fjl

core/types/block.go

@@ -71,7 +71,7 @@ func (n *BlockNonce) UnmarshalText(input []byte) error {
 type Header struct {
 	ParentHash  common.Hash    `json:"parentHash"       gencodec:"required"`
 	UncleHash   common.Hash    `json:"sha3Uncles"       gencodec:"required"`
-	Coinbase    common.Address `json:"miner"            gencodec:"required"`
+	Coinbase    common.Address `json:"miner"`
 	Root        common.Hash    `json:"stateRoot"        gencodec:"required"`
 	TxHash      common.Hash    `json:"transactionsRoot" gencodec:"required"`
 	ReceiptHash common.Hash    `json:"receiptsRoot"     gencodec:"required"`