"This is expected behavior and the only difference with previous releases is that the message has changed(previously, the message was "No report.")."
No, that is not correct. Check the screenshot again. There is a (non-functional) "Preview" Button clearly shown. This button wasn't shown before. And the message wasn't "No report" either...
Also I mention already in the title that the report source is not set... so I know why this is the case....
We are not initially setting the Report Source when showing the Report Viewer because the user can first choose between different Report Sources and then load the Report.
In the meantime, you can manually hide the whole parameters area, where the "Preview" button is located, before you set a report source to the report viewer. The visibility of the parameters area is controlled from the ParametersAreaVisible property. For example: