fix: handle missing slug in upload-coverage command

[sentry]

This PR addresses issue CLI-BN, where the upload-coverage command would crash with an unhandled ValueError: The provided slug is invalid if the --slug argument was not supplied by the user and could not be automatically resolved by a CI adapter.

The root cause was that a None value for slug was passed to encode_slug(), which then raised a ValueError when slug_with_subgroups_is_invalid(None) evaluated to True.

The fix introduces an early validation check in the upload_coverage command. If slug is None or empty, a click.ClickException is now raised with a user-friendly message, guiding the user to provide the required --slug owner/repo argument.

Fixes CLI-BN

_{This PR was automatically generated by Sentry. You can adjust this setting at any time.}

[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 1726 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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