test: added coverage to mining_basic.py

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

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")