Julien Cabieces 1c34329234 Allow expression widget to be empty so expression could be removed (#9032)
* fixes #20516 : Allow Constraint widget in attribute type form to be empty so expression could be removed

per widget modification

* correct tests

* Add the clear button on field expression widget when widget is allowed to be empty

* Change API since version to 3.6
2019-02-20 14:41:38 -05:00
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