fix: improve VulnerableSoftware comparison

[jeremylong]

Description of Change

Intermittent test failure, as seen https://github.com/dependency-check/DependencyCheck/actions/runs/18444777553/job/52549923196. Amp suggested this fix to resolve the failure.

[Copilot]

Pull Request Overview

This PR addresses intermittent test failures by improving the comparison logic in the VulnerableSoftware class to handle null and empty string values consistently.

• Adds a utility method to normalize null and empty strings for comparison
• Updates the compareTo method to use normalized values for version-related fields
• Ensures consistent comparison behavior regardless of whether fields contain null or empty strings

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