Inconsistent fallback column behavior between import/export and backoffice editing

[Dnd-Luk]

Summary
There's an inconsistency in how the fallback column is handled between the import/export strategy and the backoffice form transformer for LocalizedFallbackValue entities.
When editing LocalizedFallbackValue entities through different mechanisms, the fallback column is handled inconsistently:

• Backoffice editing: The LocalizedFallbackValueCollectionTransformer explicitly sets fallback to null when a value is provided
• Import/Export: The LocalizedFallbackValueAwareStrategy does not consistently set the fallback column to null when explicit values are provided

Steps to reproduce

• Create/edit a LocalizedFallbackValue entity through the backoffice interface
• Observe that the fallback column is set to null when providing an explicit value
• Import the same type of data using the import/export functionality
• Observe that the fallback column behavior is inconsistent

Actual Result
Import keep the fallback value

Expected Result
Import update the fallback value

Details about your environment

• OroPlatform version: 6.0.6
• PHP version: 8.3
• Database PostgreSQL version