[Form] Fixed regression in BooleanToStringTransformer from ed83752
merged branch bschussek/issue8989 (PR #9000)

This PR was merged into the 2.2 branch.

Discussion
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[Form] Fixed regression in BooleanToStringTransformer

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8989
| License       | MIT
| Doc PR        | -

Commits
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2747bdc [Form] Fixed regression in BooleanToStringTransformer from ed83752