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

CHANGELOG.md

@@ -37,6 +37,7 @@ _This release is scheduled to be released on 2024-04-01._
 - Ignore all custom css files (#3359)
 - [newsfeed] Fix newsfeed stall issue introduced by #3336 (#3361)
 - Changed `log.debug` to `log.log` in `app.js` where logLevel is not set because config is not loaded at this time (#3353)
+- added message in case where config.js is missing the module.export line PR #3383
 
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