inputMethod: Equate empty preedit string to null

If we get '' from the IM as the preedit string, it is pretty safe
to handle it as "no preedit" altogether.

(Cherry-picked from commit 286cfdc6d4)

js/misc/inputMethod.js

@@ -111,6 +111,8 @@ class InputMethod extends Clutter.InputMethod {
             return;
 
         let preedit = text.get_text();
+        if (preedit === '')
+            preedit = null;
 
         if (visible)
             this.set_preedit_text(preedit, pos, mode);