fix: output valid JSON if no games installed (#41)

Unfortunately, an empty string is not valid JSON, so if JSON mode has
been requested, make sure to output an empty JSON string if there are no
games installed while checking for updates.

nile/cli.py

@@ -165,6 +165,10 @@ def list_updates(self):
         games = self.config.get("library")
 
         if not installed_array:
+            if self.arguments.json:
+                # An empty string is not valid JSON, so create an empty but
+                # valid JSON string instead.
+                print(json.dumps(list()))
             self.logger.error("No games installed")
             return