Support adc set empty

[chalmerlowe]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @chalmerlowe, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the robustness of the Application Default Credentials (ADC) discovery process by refining how the GOOGLE_APPLICATION_CREDENTIALS environment variable is interpreted. The changes ensure that an explicitly empty value for this environment variable is correctly recognized as an indication that no explicit credentials should be used, preventing potential errors or misinterpretations during the credential loading workflow.

Highlights

• Environment Variable Handling: The os.environ.get call for GOOGLE_APPLICATION_CREDENTIALS now explicitly defaults to an empty string ("") instead of None when the variable is not set.
• Credential Path Checks: Conditional checks for the explicit_file variable have been updated from is not None to != "" to align with the new default behavior, ensuring that an empty string is correctly interpreted as the absence of an explicit credential path.
• Logging Improvements: Logging messages related to checking explicit credentials have been slightly adjusted for better formatting, enclosing the explicit_file path in single quotes.

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[gemini-code-assist]

Code Review

This pull request correctly modifies the handling of the GOOGLE_APPLICATION_CREDENTIALS environment variable to support cases where it is unset or set to an empty string. The changes ensure that such cases are gracefully ignored, allowing the authentication flow to proceed to other credential discovery methods. This is achieved by providing a default empty string to os.environ.get() and adjusting the conditional checks accordingly. Additionally, the debug logging has been improved for better security and readability by using parameterized queries. The changes are sound and improve the robustness of the credential loading process.