QGIS Server GetLegendGraphics forcing RULELABEL=TRUE where not needed (set)

[uprel]

Describe the bug
PR #32383 introduced displaying legend labels for all cases even if RULELABEL parameter was not set.
For single symbols that means duplicated legend label with layer title. If you know your layer style type then you can call its legend with RULELABEL on/off and that's fine. Problem are web clients which from GetProjectSettings do not have information about that and do not know how to adjust RULELABEL parameter.
That was not necessary in 3.4 and can be seen on pictures below (examples from Extended QWC):

QGIS and OS versions

QGIS Server 3.10.5 and current master

Additional context

Working on PR myself, will commit it shortly.