Return null for empty MultiSelection key of Dict widget

This is more consistent with the other widgets.

sao/src/view/form.js

@@ -5437,7 +5437,11 @@ function eval_pyson(value){
             },
             get_value: function() {
                 var value = this.input.val();
-                return value.map(function(e) { return JSON.parse(e); });
+                if (value && value.length) {
+                    return value.map(function(e) { return JSON.parse(e); });
+                } else {
+                    return null;
+                }
             },
             set_value: function(value) {
                 if (value) {

tryton/tryton/gui/window/view_form/view/form_gtk/dictionary.py

@@ -233,7 +233,7 @@ class DictMultiSelectionEntry(DictEntry):
 
     def get_value(self):
         model, paths = self.tree.get_selection().get_selected_rows()
-        return [model[path][0] for path in paths]
+        return [model[path][0] for path in paths] or None
 
     def set_value(self, value):
         value2path = {v: idx for idx, (v, _) in enumerate(self.selection)}