FINERACT-1266 Fix potential NPE in determineInterestPostingPeriods

[Satish11012007]

Description:
This PR addresses a potential NullPointerException in
determineInterestPostingPeriods by adding null-safety checks
for edge cases where inputs may be absent. The change improves
stability without affecting existing logic.

Key Changes:

• Added defensive null checks to prevent potential NPE
• Ensured safe handling of empty or missing inputs
• No functional changes for valid scenarios

Testing:

• Project builds successfully with Gradle
• Existing tests pass
• Manually verified affected code paths

Checklist:

• Code builds successfully
• Tests pass
• No breaking changes
• Related to FINERACT-1266 / FINERACT-932

[Satish11012007]

This PR fixes one of the potential NPE scenarios related to FINERACT-932.
Other cases tracked under the parent issue may still remain.