Geopackage raster layer name is blank in QGIS Layers panel?

I would say that QGIS should show the name as is appears in the "gpkg_contents" table and take it either from field "table_name" or perhaps rather from field "identifier". I repeated your test locally with QGIS 3.4.4 and verified your result. The names in "gpkg_contents" are OK as can be seen from the atteched image but QGIS shows blanks. I would write to QGIS mailing list and ask if this is a known issue and create a bug report if it is not.