[legacy-data-mixin 2.x] Ensure wildcard arguments get undefined treatment. Fixes #5428

[kevinpschaaf]

When checking the results of _marshalArgs for undefined values, a wildcard argument would normally always have an object and thus would not trigger a warning even if the underlying base object was undefined. In those cases check wildcard argument's base object for undefined, which would match Polymer 1's undefined rule behavior.

Reference Issue

Fixes #5428.

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[kevinpschaaf]

LGTM from sorvell based on #5522 review.