fix: etherscan api key types

hardhat.config.ts

@@ -63,14 +63,14 @@ const config: HardhatUserConfig = {
   defaultNetwork: "hardhat",
   etherscan: {
     apiKey: {
-      arbitrumOne: process.env.ARBISCAN_API_KEY,
-      avalanche: process.env.SNOWTRACE_API_KEY,
-      bsc: process.env.BSCSCAN_API_KEY,
-      mainnet: process.env.ETHERSCAN_API_KEY,
-      optimisticEthereum: process.env.OPTIMISM_API_KEY,
-      polygon: process.env.POLYGONSCAN_API_KEY,
-      polygonMumbai: process.env.POLYGONSCAN_API_KEY,
-      rinkeby: process.env.ETHERSCAN_API_KEY,
+      arbitrumOne: process.env.ARBISCAN_API_KEY || "",
+      avalanche: process.env.SNOWTRACE_API_KEY || "",
+      bsc: process.env.BSCSCAN_API_KEY || "",
+      mainnet: process.env.ETHERSCAN_API_KEY || "",
+      optimisticEthereum: process.env.OPTIMISM_API_KEY || "",
+      polygon: process.env.POLYGONSCAN_API_KEY || "",
+      polygonMumbai: process.env.POLYGONSCAN_API_KEY || "",
+      rinkeby: process.env.ETHERSCAN_API_KEY || "",
     },
   },
   gasReporter: {