bug(purchases): Purchase History shows 0 instead of blank for records with null Monthly Cost (regression after #255/PR #848)

[cristim]

Repro

1. Navigate to Purchases > Purchase History.
2. Find a purchase record that was created without a Monthly Cost value (null).
3. Observe the Monthly Cost column for that record.

Expected

The Monthly Cost cell shows blank (empty) for records where the cost is not known/set, not a misleading '0'.

Actual

(Status: Fail, awaiting re-verification. D column left blank in QA sheet -- re-test needed to confirm exact current behavior.)

Context

Issue #255 (make MonthlyCost nullable) was closed as fixed by PR #848 which added nullable MonthlyCost on PurchaseHistoryRecord. QA marked this row as Fail after the PR merged. Re-test needed to confirm the regression scenario.

Sheet reference

Bugs and improvement suggestions tab, row 519.