vconsole: reject empty layout during keymap conversion with error

A leading-dash console keymap or leading-comma X11 layout can produce an
empty layout while converting between the two formats. find_converted_keymap()
then asserts on it, allowing malformed input to abort callers such as localed.

Return EINVAL for an empty derived layout instead.

Follow-up for 6d0f502736

src/shared/vconsole-util.c

@@ -352,9 +352,11 @@ int find_converted_keymap(const X11Context *xc, char **ret) {
         int r;
 
         assert(xc);
-        assert(!isempty(xc->layout));
         assert(ret);
 
+        if (isempty(xc->layout))
+                return -EINVAL;
+
         if (xc->variant)
                 n = strjoin(xc->layout, "-", xc->variant);
         else