fix($parse): stateful interceptors override an `undefined` expression

When using `$parse` with a stateful interceptor and the expression
is `undefined`, then return the result from the interceptor

NOTE from Igor: this is not the best solution. We need to refactor
this and one-time + $interpolate code to properly fix this. @caitp
is on it. See discussion in the PR.

Closes #9821
Closes #9825