add error message if config.js appears empty after loading w require() in app.js (#3383)

from forum,
https://forum.magicmirror.builders/topic/18493/node_helper-js-is-not-working
user created own config.js, did not copy the module exports line..

this caused the js/defaults.js list of modules to be processed for
node_helpers
but the physical config.js to be loaded for the web page (hard coded in
index.html)

so user modules needing node_helper didn't have that ..

this adds a warning message in npm start output to help user resolve..
took two days to debug without message

js/app.js

@@ -115,6 +115,9 @@ function App () {
 		try {
 			fs.accessSync(configFilename, fs.F_OK);
 			const c = require(configFilename);
+			if (Object.keys(c).length === 0) {
+				Log.error("WARNING! Config file appears empty, maybe missing module.exports last line?");
+			}
 			checkDeprecatedOptions(c);
 			return Object.assign(defaults, c);
 		} catch (e) {