fix(credentials): align EnvVarStorage exists with load semantics

[harshit110927]

Description

Fixes an inconsistency in EnvVarStorage where exists() and load() used different truthiness semantics for env values.
Now exists() treats empty values as absent, matching load() behavior.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Related Issues

Fixes #5388

Changes Made

• Updated EnvVarStorage.exists() to return bool(self._read_env_value(env_var)).
• Kept load() semantics unchanged (if not value: return None).
• Added regression test test_exists_matches_load_for_empty_value to ensure consistency.

Testing

Describe the tests you ran to verify your changes:

• Unit tests pass (cd core && pytest tests/)
• Lint passes (cd core && ruff check .)
• Manual testing performed

Manual verification:

• Confirmed load() returns None for empty values.
• Confirmed exists() returns False for empty values.
• Confirmed non-empty values remain available.

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Screenshots (if applicable)

N/A (no UI changes).