fix: prevent IndexError when git remote returns no remotes

[sentry]

This PR fixes an IndexError: list index out of range that occurred in codecov_cli/helpers/versioning_systems.py.

Problem:
The empty-upload command (and other commands relying on the slug/git_service fallbacks) would crash if git remote returned only whitespace (e.g., a newline character). While p.stdout was truthy in such cases, p.stdout.decode().strip().splitlines() would result in an empty list. The subsequent unconditional access to remotes[0] then caused the IndexError.

Solution:
Added explicit checks (if not remotes: return None) after parsing the output of git remote into the remotes list. This ensures that if the git remote command yields no actual remote names, the function gracefully returns None instead of attempting to index an empty list.

Fixes CLI-H7

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[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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