gtk: Handle NULL in Editable.set_text

All the GObject string properties should handle the NULL case.

Fixes a downstream bug at https://github.com/gtk-rs/gtk4-rs/issues/981

gtk/gtkeditable.c

@@ -597,7 +597,7 @@ gtk_editable_get_text (GtkEditable *editable)
 /**
  * gtk_editable_set_text: (attributes org.gtk.Method.set_property=text)
  * @editable: a `GtkEditable`
- * @text: the text to set
+ * @text: (nullable): the text to set
  *
  * Sets the text in the editable to the given value.
  *
@@ -614,6 +614,10 @@ gtk_editable_set_text (GtkEditable *editable,
   g_object_freeze_notify (G_OBJECT (editable));
   gtk_editable_delete_text (editable, 0, -1);
   pos = 0;
+
+  if (text == NULL)
+    gtk_editable_insert_text (editable, "", 0, &pos);
+  else
     gtk_editable_insert_text (editable, text, -1, &pos);
   g_object_thaw_notify (G_OBJECT (editable));
 }