Missing property in schema update should default to previous value

## Gentics Mesh Version, operating system, or hardware.
- v1.4.x

## Problem

When omitting a property in a schema update request, Mesh sometimes assumes the default value instead of the value from the previous version.

I confirmed this is the case with the description and the label of fields. Other properties could be affected as well.

This might also affect other requests than schema updates. For schemas, this is however especially bad, because this causes a costly schema migration.

Start by reproducing a schema update request that omits the description or label, then trace the schema update request handling to find where omitted properties receive defaults. Done means omitted properties retain their previous values and schema updates no longer trigger an unnecessary costly migration.

Written by the indexing model from the issue text.