In doUpdate, if cache is null, update all fields
In doUpdate, if cache is null, update all fields

Fixes glitch where:

- existing record has non-null value
- you have a sys.db.Object with no cache (created manually, via remoting etc)
- set the value to null, try to update
- previous value remains.

Please note the private method isBinary() is now no longer used and could
be removed, though it may be useful in future.

(Cache could be null if loaded from )

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