Fix Request::getPostData() when Content-Type is null

[raicabogdan]

Hello!

• Type: bug fix
• Link to issue: [BUG]: Fix getPostData() when Content-Type is null #16804

In raising this pull request, I confirm the following:

• I have read and understood the Contributing Guidelines
• I have checked that another pull request for this purpose does not exist
• I wrote some tests for this PR
• I have updated the relevant CHANGELOG
• I have created a PR for the documentation about this change

Small description of change:
Added a check to ensure getContentType() is not null

Thanks

[niden]

Thank you @raicabogdan