Fix null pointer exception in open PR collateral calculation

[frozenflux2]

Summary

Adds a null guard for repository_tier_configuration in calculate_open_pr_collateral_score() to prevent AttributeError crashes when processing open PRs without tier configuration.

Problem

The calculate_open_pr_collateral_score() function accessed pr.repository_tier_configuration.open_pr_collateral_percentage without checking if repository_tier_configuration was None. Since repository_tier_configuration is an optional field (defined as Optional[TierConfig] = None in classes.py), this would cause an AttributeError when processing open PRs from repositories that don't have a tier configuration assigned.

Solution

• Added a null check at the start of the function
• Returns 0.0 collateral score if tier config is missing (safe default - no penalty applied)
• Logs a warning with the PR number and repository name for debugging purposes

Testing

• Manually verified the fix handles None case gracefully
• Existing tests should continue to pass as the change is additive

[frozenflux2]

@LandynDev @anderdc
This PR is ready for review.
It fixes a null pointer exception in open PR collateral calculation.
Thanks!