Merge bitcoin#34702: doc: Fix fee field in getblock RPC result

f580cc7 doc: Fix `fee` field in `getblock` RPC result (nervana21)

Pull request description:

  The `fee` field in the `getblock` RPC result (verbosity 2 and 3) may be omitted when block undo data is not available. Marking it optional in the `RPCResult` aligns the documented schema with the runtime behavior.

ACKs for top commit:
  mercie-ux:
    ACK  bitcoin@f580cc7
  satsfy:
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  instagibbs:
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  w0xlt:
    ACK f580cc7
  luke-jr:
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Tree-SHA512: e3b44a48e17e21b906967aef124688a34aea2c6af3b6df3c47693fd3002d33e824f764c0060a7ab07751b98567c29eb16f3b3c07bf2999db080ff7adfd087dfd

Author: fanquake

src/rpc/blockchain.cpp

@@ -816,7 +816,7 @@ static RPCHelpMan getblock()
                         {RPCResult::Type::OBJ, "", "",
                         {
                             {RPCResult::Type::ELISION, "", "The transactions in the format of the getrawtransaction RPC. Different from verbosity = 1 \"tx\" result"},
-                            {RPCResult::Type::NUM, "fee", "The transaction fee in " + CURRENCY_UNIT + ", omitted if block undo data is not available"},
+                            {RPCResult::Type::NUM, "fee", /*optional=*/true, "The transaction fee in " + CURRENCY_UNIT + ", omitted if block undo data is not available"},
                         }},
                     }},
                 }},