inputMethod: Do not reset invisible preedit on focus_out

If the preedit is in invisible state, the last preedit string that
was sent around is already null, so there is no need to clear the preedit
in that case.

(Cherry-picked from commit 0177560952)

js/misc/inputMethod.js

@@ -167,7 +167,7 @@ class InputMethod extends Clutter.InputMethod {
         if (this._context)
             this._context.focus_out();
 
-        if (this._preeditStr) {
+        if (this._preeditStr && this._preeditVisible) {
             // Unset any preedit text
             this.set_preedit_text(null, 0, this._preeditCommitMode);
             this._preeditStr = null;