[1.7.x] Fixed #21740 -- Allowed test client data to be an empty string

This fixes a regression introduced by 2a31d00933.
Thanks tony-zhu for the report.
Backport of f0bb3c98cc from master.
[1.7.x] Fixed #23583 -- More selectively ignored static/media roots

Fixed a regression introduced by 28efafa24c.
Thanks Michal Čihař for the report and initial patch, and
Collin Anderson and Tim Graham for the reviews.
Backport of 8b4cc9df9c from master.