Delete operation should only delete attributes which exist in the dictionary

[vkuznet]

This PR provides an additional check before deleting certain attributes in file parent dictionaries which may be omitted in the output of blockdumps API. In particular, the newly deployed Go server does not provide parent_file_id in its output, since this information is redundant (i.e. it cannot be used in migration process since these ids have no sense in destination database, and moreover python DBSMigration code does not use this information either). As such, I address this issue in this PR to make it compatible with new Go-based DBS server. We may need it during transition period when we would like to run
side by side both DBSMigration servers.

[vkuznet]

@amaltaro , I would like you to have a look at this PR as I want to make Python migration server compatible with new Go-based server where parent_file_id information is not provided (since it is redundant. The actual issue about this is here https://github.com/dmwm/dbs2go/issues/70 and we need to decide which approach to take, see my reply here https://github.com/dmwm/dbs2go/issues/70#issuecomment-1198175446 I'll comeback to this issue one I back from vacation.

[amaltaro]

Valentin, I didn't read all the discussion that you pointed out.
However, to me your change doesn't affect any behavior and it will actually have a safer key deletion. I left a suggestion in the code though.