Fix panic issues during in-circuit polynomial interpolation

[winderica]

Description

Currently, running EvaluationsVar::interpolate_and_evaluate during trusted setup (i.e., when the mode of cs is SynthesisMode::Setup) will panic, because the method internally obtains the value of an in-circuit variable by calling .value().unwrap(), but .value() always returns None during trusted setup.

This PR fixes this issue by replacing .value().unwrap() with .value().unwrap_or_default().

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