Cannot use template string with wildcard as value_field for relation widget

**Describe the bug**
When using a template string that contains a `*` wildcard (to access every item of a list) as `value_field` of a relation widget, the widget says "No options".

**To Reproduce**
1. Create a list widget, with fields `a` and `b`, and populate it
2. Create a relation widget that points to the previous list
3. Set `value_field` to `'mylist.*.a'`
4. It works
5. Set `value_field` to `'a is {{mylist.*.a}} - and b {{mylist.*.b}}'`
6. It says "No options"

**Expected behavior**
Template string should work even with wildcard.

Start by reproducing the relation widget with a template string containing wildcard values such as `mylist.*.a` and inspect the value_field handling and template parsing. Done means the relation widget presents options instead of "No options" when the template includes wildcard fields.

Written by the indexing model from the issue text.