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

Describe the changes made and why they were made. (Ignore if these details are present on the associated Apache Fineract JIRA ticket.)

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[Aman-Mittal]

Seems Ok basic null check is added.

Also need to document in PR (Exact Reproduction steps for this when it causes NPE)