accept aws polly without credential

lib/get-tts-voices.js

@@ -112,7 +112,7 @@ const getAwsVoices = async(_client, logger, credentials) => {
         credentials: assumedRoleCreds,
       });
     } else {
-      throw new Error('Missing Polly credential');
+      client = new PollyClient();
     }
     const command = new DescribeVoicesCommand({});
     const response = await client.send(command);

lib/synth-audio.js

@@ -288,7 +288,8 @@ const synthPolly = async(logger, {credentials, stats, language, voice, engine, t
         credentials: assumedRoleCreds,
       });
     } else {
-      throw new Error('Missing Polly credential');
+      // AWS RoleArn assigned to Instance profile
+      polly = new PollyClient();
     }
     const opts = {
       Engine: engine,