Add nil fix for shared_namespace

What does this MR do and why?

Handle nil gracefully to fix production issue. Followup will be handled in Ensure workspaces_agent_configs for older versi... (#541902 - closed)

Writing a test for this is challenging because

• The model does not allow nil - ActiveRecord::RecordInvalid: Validation failed: Shared namespace must be a string
• The database does not allow nil - ActiveRecord::NotNullViolation: PG::NotNullViolation: ERROR: null value in column "shared_namespace" of relation "workspaces_agent_configs" violates not-null constraint

Hence, I've skipped any tests for this for now.

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