Validation fails if tab label is empty [#3566724]

Problem/motivation

Similar to what was described and recently fixed in Drupal core in #3537774: text_textarea_with_summary widget fails if violation lacks value/summary in property path, this module's validation fails (where it didn't before) when the tab value is left empty.

Steps to reproduce

Create a field of type Bootstrap Horizontal Tab
Attempt to save it, leaving the tab label value blank
You should get a validation warning, but instead, there is a fatal error, with Warning: Undefined property: Symfony\Component\Validator\ConstraintViolation::$arrayPropertyPath

Proposed resolution

Same resolution as in #3537774: text_textarea_with_summary widget fails if violation lacks value/summary in property path

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Issue fork bootstrap_horizontal_tabs-3566724

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3566724-validation-fails-if changes, plain diff MR !26
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