fix: prevent AttributeError when ctx.obj is None in CodecovOption.get_default

[sentry]

This PR addresses an AttributeError: 'NoneType' object has no attribute 'get' that occurred when running codecov-cli --help or codecov-cli <command> --help.

The root cause was that Click's help rendering mechanism calls CodecovOption.get_default before the ctx.obj (which holds ci_adapter, versioning_system, etc.) is fully initialized. This resulted in ctx.obj being None when the code attempted to call .get() on it.

The fix adds a check for ctx.obj is not None in codecov_cli/fallbacks.py within the CodecovOption.get_default method. This ensures that the fallback logic, which relies on ctx.obj, is only executed when ctx.obj is a valid object, thus preventing the AttributeError.

Fixes CLI-GG

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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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