FINERACT-2326: Improve null-safety

[adamsaghy]

Description

As part of this PR, my intention was to achieve proper null safety in the InterestPeriod, RepaymentPeriod and ProgressiveEmiCalculator classes:

• Make sure no operation made on fields which value might be null!
• Make sure to use null-safe arithmetic operations (like MathUtil)
• Make sure for BigDecimal and Money operations to always provide the proper MathContext
• Whenever we need a "zero" value Money object, use a simple, generic way to create this object.
  I have also modified the MathUtil class and added a new "nullToZero" method which accepts MathContext as parameter and set on the Money object!

Checklist

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[somasorosdpc]

LGTM