Fix the invalid_client error when request token without the client_secret field

[glaucojunior22]

Fixes#1276

Even after the PR #1276 if the request don't have the client_secret field the response will be:
status_code: 401
{"error": "invalid_client"}
Demo image

The only way to retrieve the token is sending the client_secret field as an empty string:
Demo image

This PR is to allow the first case (request without a client_secret field) to work accordingly to the RFC specification

Reason

The Oauthlib Request object always have the client_secret param initialized as None, so getattr(request, "client_secret", "") will return None intead of the empty string even if the original request doesn't send the parameter, this is why I added the or "" at the end of this line.

Checklist

• [x ] PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• [x ] author name in AUTHORS

[n2ygk]

This looks good. Sorry for the delay in reviewing this.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.56%. Comparing base (560f84d) to head (5101903).
Report is 3 commits behind head on master.

❗ Current head 5101903 differs from pull request most recent head 14b3365. Consider uploading reports for the commit 14b3365 to get more accurate results

Additional details and impacted files

@@ Coverage Diff @@## master #1288 +/- ## ======================================= Coverage 97.56% 97.56% ======================================= Files 32 32 Lines 2132 2133 +1 ======================================= + Hits 2080 2081 +1  Misses 52 52 

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