Fixes NullPointerException in update steps

[pfeuffer]

Proposed changes

There may be v1 properties with key/value pairs, where the value is not
set. Such "pairs" led to a null pointer exception in the map function,
because no Optional could be created for the null value.
To solve this, we filter such properties without value in the beginning,
because these properties would create a null result both ways.

Fixes #1237

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