Consider undefined string attribute on label as a string

A label or separator the name of a field and no string should display the
string of the field.

issue7356
review44981002

src/view/form.js

@@ -696,7 +696,8 @@ function eval_pyson(value){
             if (this.attributes.name && record) {
                 field = record.model.fields[this.attributes.name];
             }
-            if (!this.attributes.string && field) {
+            if (!((this.attributes.string === undefined) |
+                this.attributes.string) && field) {
                 var text = '';
                 if (record) {
                     text = field.get_client(record) || '';