add empty raster query test

c.f. mapserver/mapserver#4778
Add test for https://github.com/mapserver/mapserver/issues/4800
Add missing expected result (https://github.com/mapserver/mapserver/issues/4800)
Adapt expected result to branch-6-4 behaviour (ISO-8859-1 instead of UTF-8)
add test for legend with keyimage

mapserver/mapserver#4808
Merge branch 'branch-6-4'

Conflicts:
	wxs/expected/wfs_alltypes_describefeaturetype.xml
	wxs/expected/wms_rast_empty.txt
	wxs/wms_legend.map