[MEVD] Constrain TRecord to be notnull (and/or new())

[roji]

The generic type parameter representing a record in the collection (TRecord) currently isn't constrained, and therefore can be a nullable type (e.g. Product?); that doesn't make much sense (and wouldn't work), and causes us to use bangs in various places in the codebase.

/cc @westey-m @dmytrostruk @adamsitnik

[adamsitnik]

Fixed with #11467