Thank you @Saurabh Sharma
Symptom:
Web Activity and REST connector have error in GET request.
Error: Details { 'errors' : [ { 'message' : 'Content type '' not supported', 'type' : 'HttpMediaTypeNotSupportedError' } ] }
Cause:
ADF drops the Content-type header from a GET request if ‘request body’ is not specified, so per REST semantics, it should not have Content-type header enforced in a REST API because Content-Type entity-header field indicates the media type of the entity-body sent to the recipient
Resolution:
Change the receiving API setting content-type as optional