[TableGen] Fix convertFromStorage for OptionalAttr

    OptionalAttr is just wrapping around the actual attribute; so it should just use
    the actual attribute's `convertFromStorage` to read the value and wrap it around
    with `Optional<>` to return. Previously it was mandating how the actual attribute
    reads the value with `{0}.getValue()`.

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PiperOrigin-RevId: 241762355