Add default to block_time_ms

Author: MegaRedHand

config/runtime.exs

@@ -166,6 +166,8 @@ block_time_ms =
     "mainnet" -> 12_000
     "sepolia" -> 100
     "holesky" -> 24_000
+    # Default to mainnet
+    _ -> 12_000
   end
 
 config :lambda_ethereum_consensus, LambdaEthereumConsensus.Telemetry,