FINERACT-2710: Fix NPE downloading the recurring-deposit template without a term type

[oluexpert99]

 - GET /v1/recurringdepositaccounts/downloadtemplate returned HTTP 500 on any tenant
    holding a recurring-deposit product with no minimum-deposit-term type:
    RecurringDepositProductSheetPopulator.populate writes the min-deposit-term-type cell
    through an unguarded product.getMinDepositTermType().getValue(), so a null term type
    throws and the whole template download fails. The template is unusable for the entire
    tenant and there is no API-side workaround.
  - The field is optional on the product, and every sibling optional field in the same loop
    (min/max deposit amount, min/max deposit term, lock-in period, pre-closure penal
    interest) is already null-guarded. The fixed-deposit product sheet populator guards the
    equivalent field too, so this is an inconsistency between two sibling populators rather
    than a deliberate requirement.
  - Guard the write like its siblings: a product without a minimum-deposit-term type leaves
    that lookup cell blank instead of failing the download. Nothing changes for products
    that do carry one.
  - Add a unit test (RecurringDepositProductSheetPopulatorTest) covering a product whose
    term type is null: it fails on the unfixed code with the exact production NPE and
    asserts the product sheet is still written.

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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