fix: blockchain.block.header returns plain hex string when cp_height is zero

[eynhaender]

Per the Electrum protocol spec, blockchain.block.header must return a
plain hex string when cp_height is 0. The shared blockchain_block_headers()
implementation was unconditionally wrapping the result in an object,
causing Electrum clients (e.g. Sparrow) to fail parsing the response.

Two fixes in blockchain_block_headers():

• Add early return for the no-proof singular case that sends a plain
  std::string via send_result() instead of building an object_t.
• Fix the pre-v1.6 proof dict to use "header" for the singular case;
  "hex" is only correct for the plural (concatenated) response.