rpc: fix crash in getcompactsketch

This originates in 8723deb which has no
PR associated with it. We've really gotta stop putting thousands of
unreviewed commits into this project and rebasing the history away..

Author: apoelstra

src/rpc/mining.cpp

@@ -1527,6 +1527,10 @@ static RPCHelpMan getcompactsketch()
     CDataStream ssBlock(block_bytes, SER_NETWORK, PROTOCOL_VERSION);
     ssBlock >> block;
 
+    if (block.vtx.empty()) {
+        throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "Cannot obtain sketch of empty block.");
+    }
+
     CBlockHeaderAndShortTxIDs cmpctblock(block, true);
 
     CDataStream ssCompactBlock(SER_NETWORK, PROTOCOL_VERSION);