Add support to retrieve genesis if empty hash is supplied (#238)

- This is used by the block indexer in the "blockcore-bitcore" repo.

Author: sondreb

src/Blockcore/Consensus/Chain/ChainIndexer.cs

@@ -221,6 +221,12 @@ public virtual ChainedHeader GetHeader(uint256 id)
         {
             lock (this.lockObject)
             {
+                // If node is looking for zero, we'll return the genesis block.
+                if (id == uint256.Zero)
+                {
+                    return this.blocksByHeight[0];
+                }
+
                 ChainedHeader result;
                 this.blocksById.TryGetValue(id, out result);
                 return result;