download-unpacked: fix strip-top-level-dir if unset and nothing should be stripped

This fixes a regression that was introduced in
commit d5f5d399.
Status Job ID Name Coverage
  External
running SlapOS.Eggs.UnitTest-Boxiang.Python2

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running SlapOS.Eggs.UnitTest-Boxiang.Python3

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passed SlapOS.Eggs.UnitTest-Master.Python2

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passed SlapOS.Eggs.UnitTest-Boxiang.Python2

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passed SlapOS.Eggs.UnitTest-Master.Python3

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