Substrait string concat with more than two arguments fails conversion to Calcite

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The Substrait concat function is variadic so allows more than two arguments. When converting a Substrait concat expression with three (or more) arguments to Calcite, an error occurs since Calcite's corresponding || operator only allows two operands:

java.lang.AssertionError: wrong operand count 3 for ||
	at org.apache.calcite.util.Litmus.lambda$static$0(Litmus.java:31)
	at org.apache.calcite.sql.SqlBinaryOperator.validRexOperands(SqlBinaryOperator.java:229)
	at org.apache.calcite.rex.RexCall.<init>(RexCall.java:93)
	at org.apache.calcite.rex.RexBuilder.makeCall(RexBuilder.java:293)
	at org.apache.calcite.rex.RexBuilder.makeCall(RexBuilder.java:279)
	at io.substrait.isthmus.expression.ExpressionRexConverter.visit(ExpressionRexConverter.java:433)

This could be resolved by modifying the expression during conversion to be nested / chained expressions, each with only two arguments. For example, concat(a, b, c) becomes concat(concat(a, b), c). This results in the expected Calcite (and SQL) output.