olm: Also initialize all fields when decoding Olm messages.

As a precaution.
10 jobs for master in 5 minutes and 31 seconds (queued for 6 seconds)
Child pipeline (parent)
Status Name Job ID Coverage
  Build
passed build:python #14074

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  Test
passed test:python: [3.6] #14077

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passed test:python: [3.7] #14078

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passed test:python: [3.8] #14079

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passed test:python: [3.9] #14080

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  Dist
passed dist:python:sdist #14081

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passed dist:python:wheel: [3.6] #14082

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passed dist:python:wheel: [3.7] #14083

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passed dist:python:wheel: [3.8] #14084

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passed dist:python:wheel: [3.9] #14085

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