FINERACT-1897: Fix NPE when requestedShares is null during share acco…

[SamaSVM]

Description

Null Long unboxing in arithmetic comparison (requestedShares < minimumClientShares) caused NPE before the validation error list was checked, returning HTTP 500 instead of HTTP 400. Added null guard on both min/max comparisons so validation completes normally and the client receives a proper 400 response.

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