Import treats cleared values for culture invariant meta fields incorrectly

Description

The following behavior, around since Episerver Commerce 9.x, causes issues with various integration scenarios.

1. Verify a value for a culture invariant meta field in the product catalog.
2. Import a file which includes an empty value for the culture invariant field, but for a language other than the catalog default language (e.g., default language = en, import file has empty value included for culture invariant field in sv).
3. The value for culture invariant field is cleared.

This is contrary to the treatment of culture invariant fields in the API and for changed values in the import (e.g. the field has initial value "foo" for the culture invariant field and import for language sv contains value "bar." After import, the value is still "foo").

This behavior is changed. Step 3 above results in the culture-specific field being cleared, and the culture-invariant field retaining its original value.