Be more defensive when loading mimic config

Default config will generally always have a mimic block but in
combination with newer Mycroft variants a more defensive approach is
probably good

mycroft/tts/mimic_tts.py

@@ -39,7 +39,7 @@ def get_mimic_binary():
     Returns:
         (str) path of mimic executable
     """
-    config = Configuration.get().get("tts").get("mimic")
+    config = Configuration.get().get("tts", {}).get("mimic")
 
     bin_ = config.get("path",
                       os.path.join(MYCROFT_ROOT_PATH, 'mimic', 'bin', 'mimic'))