[CosmosDb] Handling null Cosmos DB key

[fhibf]

Description

Changes to better handle null Cosmos DB key.
That's a valid scenario when using Managed Identity, and a null (instead of empty) key is provided as part of the Cosmos DB configuration.

Related issues

Addresses AB#184099.

Testing

Tested using unit tests. Simulating a scenario found in one of our internal test accounts.

FHIR Team Checklist

• Update the title of the PR to be succinct and less than 65 characters
• Add a milestone to the PR for the sprint that it is merged (i.e. add S47)
• Tag the PR with the type of update: Bug, Build, Dependencies, Enhancement, New-Feature or Documentation
• Tag the PR with Open source, Azure API for FHIR (CosmosDB or common code) or Azure Healthcare APIs (SQL or common code) to specify where this change is intended to be released.
• Tag the PR with Schema Version backward compatible or Schema Version backward incompatible or Schema Version unchanged if this adds or updates Sql script which is/is not backward compatible with the code.
• When changing or adding behavior, if your code modifies the system design or changes design assumptions, please create and include an ADR.
• CI is green before merge
• Review squash-merge requirements

Semver Change (docs)

Patch|Skip|Feature|Breaking (reason)