rpc: fix OpenRPC hex pattern to allow empty strings #36462

The OpenRPC schema from getopenrpcinfo declares every STR_HEX value with the pattern ^[0-9a-fA-F]+$. That pattern rejects the empty string, but the node both returns and accepts empty hex. For example:

• scriptSig.hex is "" for native segwit input (decoderawtransaction, getrawtransaction, etc)
• decodescript "" succeeds

So a client that validates traffic against the schema rejects valid requests and responses.

This changes the pattern to ^([0-9a-fA-F]{2})*$ (zero or more hex-encoded bytes)

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36462
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36462.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36462#author-ralph

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lgtm ACK 4d7e00d36bf0d287dc241c3aae3852f3d799ed08

ACK 4d7e00d36bf0d287dc241c3aae3852f3d799ed08

ACK 4d7e00d36bf0d287dc241c3aae3852f3d799ed08

In theory, one could have size set (in the docs and the schema), if the size is hard and known, but this is probably a massively larger change.

Backported to 32.x in #36427.