Quote optional boolean fields

https://github.com/canonical/inference-snaps-testing/blob/0dc80e4ab03f453de3e970e0a1cdff5489f6c659/.github/workflows/deepseek-r1.yaml#L84

This script will be maintained by a user, not the developer of the tests. To make it easier to use, perhaps we can quote this line, passing an empty string into the tests, avoiding the inline conditional. The tests should handle unset values, not the workflow file.

Start with line 84 of .github/workflows/deepseek-r1.yaml and inspect how the optional boolean value is passed into the tests. Update the workflow so an unset value is passed as an empty string, then verify that the tests handle unset values as intended.

Written by the indexing model from the issue text.