erp5_core: html viewer: do not drop empty element

They are often used to achieve surprising styling
52 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

01:03:16

passed ERP5.PerformanceTest-Master

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failed ERP5.UnitTest-Master

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failed ERP5.UnitTest-Master.Medusa

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passed SlapOS.Eggs.UnitTest-Master.Python2

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passed SlapOS.Eggs.UnitTest-Master.Python3

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failed ERP5.UnitTest-Master

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failed ERP5.UnitTest-Master

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failed ERP5.UnitTest-Master

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failed ERP5.UnitTest-Master

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failed ERP5.UnitTest-Master

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failed ERP5.UnitTest-Master

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failed ERP5.UnitTest-Master

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failed ERP5.UnitTest-Master

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failed ERP5.UnitTest-Master

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failed ERP5.UnitTest-Master

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