user-with-full-skel.bbclass: Fix empty USERNAME case

Signed-off-by: Andreas Müller <schnitzeltony@gmail.com>

classes/user-with-full-skel.bbclass

@@ -16,11 +16,13 @@ SKEL_INIT_MARKER = "force-skel-full-init"
 
 # user recipe part
 pkg_postinst_${PN}() {
-    if [ -n "$D" ]; then
+    if [ -n "$D" -a -n "${USERNAME}" ]; then
         touch $D/home/${USERNAME}/${SKEL_INIT_MARKER}
     fi
 }
 
+do_package[vardeps] += "USERNAME"
+
 # image recipe part
 ROOTFS_POSTPROCESS_COMMAND += "postinst_copy_skel; "