fix deserialization issue with null workspace refs

[mjkkirschner]

Purpose

reported here:
DynamoDS/DynamoRevit#2869

no jira ticket, fixed with an extra null check and a test, I have no idea how this became null to begin with though.

Declarations

Check these if you believe they are true

• The codebase is in a better state after this PR
• Is documented according to the standards
• The level of testing this PR includes is appropriate
• User facing strings, if any, are extracted into *.resx files
• All tests pass using the self-service CI.
• Snapshot of UI changes, if any.
• Changes to the API follow Semantic Versioning and are documented in the API Changes document.
• This PR modifies some build requirements and the readme is updated

Release Notes

Fix issue loading file if it contains null list of workspace references.

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[RobertGlobant20]

LGTM