FINERACT-2689: Fix NPE in deposit withholding tax when net interest posted is zero

[oluexpert99]

• Closing (or posting interest on) a withholding-tax deposit account threw a 500
  NullPointerException whenever the net interest posted on the account was zero:
  SavingsAccount.createWithHoldTransaction / updateWithHoldTransaction guard only on
  the tax group and then evaluate amount.compareTo(BigDecimal.ZERO), but amount can be
  null. SavingsAccountTransactionSummaryWrapper.calculateTotalInterestPosted returns
  total.getAmountDefaultedToNullIfZero() -- i.e. null -- when there are no net
  interest-posting transactions, and that null is passed straight into both helpers.
  - The common real-world trigger is premature-closing a short-term fixed deposit whose
  pre-closure penalty rate is greater than or equal to the chart rate: the effective
  interest is clamped to zero (FixedDepositAccount.getEffectiveInterestRateAsFraction),
  so postPreMaturityInterest posts no interest transaction, and the withholding-tax pass
  then runs with a null interest total. For a withholding-tax account the tax group is
  non-null, so the first clause passes and amount.compareTo throws; the whole command
  rolls back and the account cannot be closed.
  - Null-guard amount in both createWithHoldTransaction and updateWithHoldTransaction: a
  null (or non-positive) interest total means there is nothing to withhold tax on, so
  both no-op and return false. No behaviour change when interest is actually posted -- a
  non-null positive amount takes the same path as before.
  - Add a unit test (SavingsAccountWithHoldTaxNullAmountTest) that sets a non-null tax
  group so the guard is exercised rather than short-circuited, and asserts that a null
  or zero amount no longer throws and adds no withholding transaction.

Description

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