Cross obj null

When evaluating formula like `Account.Name` in salesforce, if `Account` is `null`, then `Account.Name` is treated as `null`. Currently, this `pixel9:liquidframeworks` will throw an error under the same conditions.

Changes
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This PR will cause the expression `Account.Name` to be evaluated as `scope.Account && scope.Account.Name || null`, thereby returning `null` when there is no `Account`. This behavior also applied to deeply nested object graphs.