Does Qgis server support SLD and SLD_BODY parameters in GetMap request?
I'm trying to give a valid .sld to qgis server but it returns empty image in response.
It depends on your QGIS version. If you use QGIS server 3 then the SLD support was dropped due to another change https://issues.qgis.org/issues/19795.
With the suppression of the class QgsHttpTransaction, the support of SLD parameter has been removed.
The support is coming back with this pull request https://github.com/qgis/QGIS/pull/7859.