fix: prevent ValueError when slug is missing with --handle-no-reports-found

[sentry]

This PR addresses an issue where the CLI would crash with a ValueError: The provided slug is invalid when do-upload was called with --handle-no-reports-found enabled, but no --slug argument was provided.

The root cause was that when no coverage reports were found, the do_upload_logic function would attempt to call upload_completion_logic to trigger notifications, passing slug=None. The encode_slug function, called by upload_completion_logic, does not handle None gracefully and raises a ValueError.

The fix introduces a check in do_upload_logic to ensure that slug is not None before attempting to call upload_completion_logic in the --handle-no-reports-found fallback path. If slug is None, a warning is logged, and the upload completion step is skipped, preventing the crash and providing a more informative output to the user.

Fixes CLI-FT

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
723	1	722	1

View the full list of 1 ❄️ flaky test(s)

api.temp.calculator.test_calculator::test_divide

Flake rate in main: 100.00% (Passed 0 times, Failed 1625 times)

Stack Traces | 0.001s run time

def
                test_divide():
                > assert Calculator.divide(1, 2) == 0.5
                E assert 1.0 == 0.5
                E + where 1.0 = <function Calculator.divide at 0x104c9eb90>(1, 2)
                E + where <function Calculator.divide at 0x104c9eb90> = Calculator.divide
                .../temp/calculator/test_calculator.py:30: AssertionError

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