fix: default to empty string for mnemonic

truffle-config.js

@@ -8,7 +8,7 @@ const providerWithMnemonic = (mnemonic, rpcEndpoint) =>
   new HDWalletProvider(mnemonic, rpcEndpoint);
 
 const infuraProvider = network => providerWithMnemonic(
-  process.env.MNEMONIC,
+  process.env.MNEMONIC || '',
   `https://${network}.infura.io/${process.env.INFURA_API_KEY}`
 );