Merge bitcoin/bitcoin#27603: test: added coverage to mining_basic.py

a7b46a1fea test: added coverage to mining_basic.py (kevkevin)

Pull request description:

  Included a test that checks if we call submitblock with block.vtx.empty() then it throws an rpc deserialization error, currently we only test if !block.vtx->IsCoinBase() throws an rpc deserialization error

  I've tested to make sure this actually doing what I intended by breaking up this if block into two if blocks with different error messages and running the functional test
  https://github.com/bitcoin/bitcoin/blob/322ec63b01499c1ec52d3912ee382ebd59f2366b/src/rpc/mining.cpp#L963

  This change should increase the test coverage for the `submitblock()` rpc in `./src/rpc/mining.cpp`

ACKs for top commit:
  theStack:
    ACK a7b46a1fea

Tree-SHA512: 4078cb1fa879cc9e34438319f73085b521b90a5a95348b23e494cf8e5ac792ec426bc0e1a63e949645e16afebe54c5f35a194f02e20b7273871163d89a5c44e6

test/functional/mining_basic.py

@@ -145,6 +145,7 @@ class MiningTest(BitcoinTestFramework):
         assert_template(node, bad_block, 'bad-cb-missing')
 
         self.log.info("submitblock: Test invalid coinbase transaction")
+        assert_raises_rpc_error(-22, "Block does not start with a coinbase", node.submitblock, CBlock().serialize().hex())
         assert_raises_rpc_error(-22, "Block does not start with a coinbase", node.submitblock, bad_block.serialize().hex())
 
         self.log.info("getblocktemplate: Test truncated final transaction")