Resolve default namespaces properly in semanticSearch test

[austin-denoble]

Problem

Hard-coded check of namespaces[""] failing on upgrade to latest API version. Needs to check for either "" or "__default__".

Solution

Make sure we're checking for both default namespaces everywhere.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Infrastructure change (CI configs, etc)
• Non-code change (docs, etc)
• None of the above: (explain here)

Test Plan

n/a