Update 'Using mailchimp' readme section

[jose-bittacora]

While using your package, I followed the instructions, but I was always getting empty responses and I couldn't get any information from getLastError() or getLastResponse(). I ran the code step by step and found that if NEWSLETTER_ENDPOINT is set to an empty string as the documentation states, the Mailchimp endpoint isn't built correctly. The solution is to set that value to null instead.

[Arnwaltur]

Hi,
+1 for this Mailchimp patch

[Applejack21]

Bump for this to be merged. Fixed for me as using 'endpoint' => env('NEWSLETTER_ENDPOINT', ''), didn't subscribe the user to the mailing list but 'endpoint' => env('NEWSLETTER_ENDPOINT', null), does.

[code23-isaac]

+1 this fixes mailchimp issues

[DanDvoracek]

For anyone stumbling on this page, make sure to NOT put a NEWSLETTER_ENDPOINT value in your .env on top of it. If you specify it empty/without a value there you might have silent mistakes too.

[Datalink-Wouter]

Omg legends! I couldn't find why every endpoint returned a False, appears to be this NEWSLETTER_ENDPOINT thing.

[niekappartmedia]

@freekmurze is this something you can somehow merge because its something i stumbeld over with the simple fix of setting the value to null.

[freekmurze]

Thanks!