Allow Constraint widget in attribute type form to be empty so expression could be removed

[troopa81]

Description

Backport of #9032

Checklist

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[troopa81]

@3nids do you mid merging this PR ? It's just a backport of #9032 that you have merged few days ago

[3nids]

well, the dox are not correct then. should be since QGIS 3.4.?

[troopa81]

well, the dox are not correct then. should be since QGIS 3.4.?

Not sure what you mean with the dox?

[m-kuhn]

He was referring to this
https://github.com/qgis/QGIS/pull/9229/files#diff-50164c89f27f4653caaa7c9d1662d96eR89

[troopa81]

OK, can you confirm that I have to change the version in both masterand release_3-4 branch to Since 3.4

[m-kuhn]

It should be changed to \since 3.4.6.

[troopa81]

I make the correction and created the PR #9480 to make the correction in master.

And then, I will backport the modification in 3.6