Avoid incorrect warning about the Destination package

[Vassiliy-Kudryashov]

Description

Actually we don't show textfield to specify result package for tests. Thus in model is't just an empty string. The fix don't procude the warning in case of empty string.

Fixes #1915

Manual tests

See the steps from #1915, actually the issue should appear always when you generate tests.

Self-check list

• I've set the proper labels for my PR (at least, for category and component).
• PR title and description are clear and intelligible.
• I've added enough comments to my code, particularly in hard-to-understand areas.
• The functionality I've repaired, changed or added is covered with automated tests.
• Manual tests have been provided optionally.
• The documentation for the functionality I've been working on is up-to-date.