fix(core): output nullability of IfThen depends on all possible outputs

For an IfThen expression, the output should be nullable if any of the possible
outputs is nullable

Author: bvolpato-dd

core/src/main/java/io/substrait/expression/Expression.java

@@ -584,7 +584,13 @@ abstract static class IfThen implements Expression {
     public abstract Expression elseClause();
 
     public Type getType() {
-      return elseClause().getType();
+      Type elseType = elseClause().getType();
+
+      // If any of the clauses are nullable, the whole expression is also nullable.
+      if (ifClauses().stream().anyMatch(clause -> clause.then().getType().nullable())) {
+        return TypeCreator.asNullable(elseType);
+      }
+      return elseType;
     }
 
     public static ImmutableExpression.IfThen.Builder builder() {

core/src/test/java/io/substrait/type/proto/IfThenRoundtripTest.java

@@ -0,0 +1,45 @@
+package io.substrait.type.proto;
+
+import static io.substrait.expression.proto.ProtoExpressionConverter.EMPTY_TYPE;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import io.substrait.TestBase;
+import io.substrait.expression.Expression;
+import io.substrait.expression.ExpressionCreator;
+import io.substrait.expression.proto.ExpressionProtoConverter;
+import io.substrait.expression.proto.ProtoExpressionConverter;
+import java.util.Arrays;
+import org.junit.jupiter.api.Test;
+
+public class IfThenRoundtripTest extends TestBase {
+
+  @Test
+  void ifThenNotNullable() {
+    final Expression.IfThen ifRel =
+        b.ifThen(
+            Arrays.asList(
+                b.ifClause(ExpressionCreator.bool(false, false), ExpressionCreator.i64(false, 1))),
+            ExpressionCreator.i64(false, 2));
+    assertFalse(ifRel.getType().nullable());
+
+    var to = new ExpressionProtoConverter(null, null);
+    var from = new ProtoExpressionConverter(null, null, EMPTY_TYPE, protoRelConverter);
+    assertEquals(ifRel, from.from(ifRel.accept(to)));
+  }
+
+  @Test
+  void ifThenNullable() {
+    final Expression.IfThen ifRel =
+        b.ifThen(
+            Arrays.asList(
+                b.ifClause(ExpressionCreator.bool(true, false), ExpressionCreator.i64(true, 1))),
+            ExpressionCreator.i64(false, 2));
+    assertTrue(ifRel.getType().nullable());
+
+    var to = new ExpressionProtoConverter(null, null);
+    var from = new ProtoExpressionConverter(null, null, EMPTY_TYPE, protoRelConverter);
+    assertEquals(ifRel, from.from(ifRel.accept(to)));
+  }
+}