Use /var/empty as the homedir for _apt

Thanks to Axel Beckert

debian/apt.postinst

@@ -27,8 +27,8 @@ case "$1" in
 	fi
 
         # add unprivileged user for the apt methods
-        adduser --force-badname --system --no-create-home \
-            --quiet _apt || true
+        adduser --force-badname --system -home /var/empty \
+            --no-create-home --quiet _apt || true
         chown -R _apt:root \
             /var/lib/apt/lists \
             /var/cache/apt/archives