Fix non-nullable type w/ ? warnings

[alancai98]

Issue #, if available: fixes #154

Description of changes: PIG-generated code was previously always adding a ? to non-nullable values for builder function constructor and .toIonElement calls in which there was a variadic list of parameters. This is currently a warning in Kotlin 1.6 but causes in error in Kotlin 1.7 and later.

Example for builder function constructor:

        /**
         * Creates an instance of [PartiqlBasic.Expr.Call].
         */
        fun call(
            name: String,
            args0: Expr,
            vararg args: Expr,
            metas: MetaContainer = emptyMetaContainer()
        ): PartiqlBasic.Expr.Call =
            PartiqlBasic.Expr.Call(
                name = name?.asPrimitive(),   <- `name` is not nullable
                args = listOf(args0) + args.toList(),
                metas = newMetaContainer() + metas
            )

With the fix, this will now be:

        /**
         * Creates an instance of [PartiqlBasic.Expr.Call].
         */
        fun call(
            name: String,
            args0: Expr,
            vararg args: Expr,
            metas: MetaContainer = emptyMetaContainer()
        ): PartiqlBasic.Expr.Call =
            PartiqlBasic.Expr.Call(
                name = name.asPrimitive(),
                args = listOf(args0) + args.toList(),
                metas = newMetaContainer() + metas
            )

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[alancai98]

fixes the builder function call

[alancai98]

fixes the .toIonElement() call

[dlurton]

Thanks!