Fix Extra Packages column in systems list

[wweellddeerr]

What does this PR change?

It was showing wrong value when extra packages count is zero.

GUI diff

No difference.

• [x] DONE

Documentation

• No documentation needed: bug fix.

• [x] DONE

Test coverage

ℹ️ If a major new functionality is added, it is strongly recommended that tests for the new functionality are added to the Cucumber test suite

• No tests: add explanation

• No tests: already covered

• Unit tests were added

• Cucumber tests were added

• [x] DONE

Links

Issue(s): # Port(s): # add downstream PR(s), if any

• [x] DONE

Changelogs

Make sure the changelogs entries you are adding are compliant with https://github.com/uyuni-project/uyuni/wiki/Contributing#changelogs and https://github.com/uyuni-project/uyuni/wiki/Contributing#uyuni-projectuyuni-repository

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• [ ] No changelog needed

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Re-run a test

If you need to re-run a test, please mark the related checkbox, it will be unchecked automatically once it has re-run:

• [ ] Re-run test "changelog_test"
• [ ] Re-run test "backend_unittests_pgsql"
• [ ] Re-run test "java_pgsql_tests"
• [ ] Re-run test "schema_migration_test_pgsql"
• [ ] Re-run test "susemanager_unittests"
• [ ] Re-run test "javascript_lint"
• [ ] Re-run test "spacecmd_unittests"

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