fix(sync): treat optional empty responses as no data

[hxu]

Summary

• Treat Garmin endpoints that legitimately return empty or no-content responses as no data.
• Use the optional JSON response path for sync calls where an empty response should not fail the task.
• Avoid sync failures caused by optional endpoints returning empty bodies.

This should build on #21 to actually use the new endpoints and not break the task when we encounter an empty response

Tests

• cargo test