`DocumentSourceCollStats::createFromBson()` passes a `SerializationContext` object pulled from the expression context into the `IDLParserContext` object used by `DocumentSourceCollStatsSpec::parse()`, but the expression context may or may not contain a valid `SerializationContext` object.  An invalid `SerializationContext` object can be default constructed on construction of an `ExpressionContext` object as `ExpressionContext` objects are used in several different ways.  When an `ExpressionContext` object is constructed via an `AggregateCommandRequest`, it should always contain a valid `SerializationContext` object, so we should assert this when `DocumentSourceCollStatsSpec::parse()` is called in order to avoid serializing a collStats reply incorrectly yet failing silently.