Change Safe validation schema to allow null Safe version

[hectorgomezv]

Problem
Some validation errors are being caused by Safes with a null version field.
Example here.
More context here.

Solution
Once the Safe{Core} team confirms this is a rare but expected case, make version field nullable in the Safe validation schema. Adjust OpenAPI schema and/or tests if necessary.

[fmrsabino]

I don't think this is a bug. We can consider that these Safes are invalid also. One of the goals of the Safe Client Gateway is to provide to the UI clients a formatted response that we support. Right now we support safes that have a valid version.

[hectorgomezv]

Closing the issue in favor of #569