FINERACT-2489: Fix NPE for missing charts in Recurring Deposit Product

[Dhanno98]

Description

Implements FINERACT-2489.

Problem

While creating a Recurring Deposit Product using POST /v1/recurringdepositproducts, the request fails with HTTP 500 Internal Server Error when the charts parameter is omitted.

Current Behavior

The validation flow effectively requires charts as required. However, when it is missing a NullPointerException occurs inside DepositProductDataValidator.validateChartsData() before the validation framework can return a proper validation error.
So the users receive a 500 response instead of a 400 along with a meaningful validation message.

Why this happens

Unlike optional parameters, the code path for charts does not check parameterExists() before validation, which indicates the parameter is mandatory.
DataValidatorBuilder.notNull().jsonArrayNotEmpty() is already present but execution proceeds further and triggers NPE.

What this PR does

• Add a null guard to prevent NPE.
• Allow the existing validator to correctly return a 400 Bad Request with a proper validation message consistent with the rest of the mandatory fields when they are found missing.
• Updates API documentation to move charts from Optional Fields to Mandatory Fields.

Result after fix

If charts is missing, the API now returns a consistent validation error, similar to other mandatory fields.
No functional behavior is changed beyond improving error handling and documentation alignment.

Checklist

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[Dhanno98]

Rebased the branch onto the latest develop and force-pushed. Thank you!

[Dhanno98]

I checked the failed job and it appears that the failure in verify (build-core, main) is related to the unresolved org.eclipse.swt.${osgi.platform} dependency. I can see similar failures in a few other recent PRs as well.

I have not modified any build or dependency configuration in this PR but please let me know if you think I should investigate anything further or rebase once this is resolved in develop.
Thank you.