imwayland: Fix clearing of preedit text in webkitgtk

Fixes webkitgtk misbehaviour as outlined in https://gitlab.gnome.org/GNOME/gtk/issues/1316#note_312942 , which was introduced in 49b17e6c.
The preedit will be cleared on exit only if it is already present.

modules/input/imwayland.c

@@ -654,8 +654,11 @@ gtk_im_context_wayland_focus_out (GtkIMContext *context)
   commit_state (context_wayland);
 
   /* after disable, incoming state changes won't take effect anyway */
+  if (context_wayland->current_preedit.text)
+    {
       text_input_preedit (global, global->text_input, NULL, 0, 0);
       text_input_preedit_apply (global);
+    }
 
   global->current = NULL;
 }