Dexcom empty data guard

[bjorkert]

What & Why

dexShare?.fetchData() could return an empty array even when the network call succeeds.
Our previous guard only checked for nil, so subscripting data[0] would crash with “index out of range”.

Changes

• Extend the guard clause to guard let data = result, !data.isEmpty
• Update the log message to “Received empty data array from Dexcom” for clarity.

How to test

1. Point the Dexcom share stub/service at a response that returns [].
2. Launch the app and trigger a download.
3. Verify:
   • No crash occurs.
   • Log output shows the new message.
   • The fallback to webLoadNSBGData() is executed.

Notes

No functional changes beyond the safety check.

[dnzxy]

Absolutely sane and surgical check update. LGTM.