SQL: Fix issue with DataType for CASE with NULL (elastic#46173)

Previously, if the DataType of all the WHEN conditions of a CASE
statement is NULL, then it was set to NULL even if the ELSE clause
has a non-NULL data type, e.g.:
```
CASE WHEN a = 1 THEN NULL
           WHEN a = 5 THEN NULL
ELSE 'foo'
```

Fixes: elastic#46032

Author: matriv

x-pack/plugin/sql/src/main/java/org/elasticsearch/xpack/sql/expression/predicate/conditional/Case.java

@@ -58,6 +58,7 @@ public DataType dataType() {
                 for (IfConditional conditional : conditions) {
                     dataType = DataTypeConversion.commonType(dataType, conditional.dataType());
                 }
+                dataType = DataTypeConversion.commonType(dataType, elseResult.dataType());
             }
         }
         return dataType;

x-pack/plugin/sql/src/test/java/org/elasticsearch/xpack/sql/expression/predicate/conditional/CaseTests.java

@@ -6,19 +6,23 @@
 package org.elasticsearch.xpack.sql.expression.predicate.conditional;
 
 import org.elasticsearch.xpack.sql.expression.Expression;
+import org.elasticsearch.xpack.sql.expression.Literal;
 import org.elasticsearch.xpack.sql.expression.function.scalar.FunctionTestUtils;
 import org.elasticsearch.xpack.sql.expression.predicate.operator.comparison.Equals;
 import org.elasticsearch.xpack.sql.tree.AbstractNodeTestCase;
 import org.elasticsearch.xpack.sql.tree.NodeSubclassTests;
 import org.elasticsearch.xpack.sql.tree.Source;
 import org.elasticsearch.xpack.sql.tree.SourceTests;
+import org.elasticsearch.xpack.sql.type.DataType;
 
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.List;
 import java.util.Objects;
 
 import static org.elasticsearch.xpack.sql.expression.function.scalar.FunctionTestUtils.randomIntLiteral;
 import static org.elasticsearch.xpack.sql.expression.function.scalar.FunctionTestUtils.randomStringLiteral;
+import static org.elasticsearch.xpack.sql.tree.Source.EMPTY;
 import static org.elasticsearch.xpack.sql.tree.SourceTests.randomSource;
 
 /**
@@ -77,6 +81,42 @@ public void testReplaceChildren() {
         assertEquals(new Case(c.source(), newChildren), c.replaceChildren(newChildren));
     }
 
+    public void testDataTypes() {
+        // CASE WHEN 1 = 1 THEN NULL
+        // ELSE 'default'
+        // END
+        Case c = new Case(EMPTY, Arrays.asList(
+            new IfConditional(EMPTY, new Equals(EMPTY, Literal.of(EMPTY, 1), Literal.of(EMPTY, 1)), Literal.NULL),
+            Literal.of(EMPTY, "default")));
+        assertEquals(DataType.KEYWORD, c.dataType());
+
+        // CASE WHEN 1 = 1 THEN 'foo'
+        // ELSE NULL
+        // END
+        c = new Case(EMPTY, Arrays.asList(
+            new IfConditional(EMPTY, new Equals(EMPTY, Literal.of(EMPTY, 1), Literal.of(EMPTY, 1)), Literal.of(EMPTY, "foo")),
+            Literal.NULL));
+        assertEquals(DataType.KEYWORD, c.dataType());
+
+        // CASE WHEN 1 = 1 THEN NULL
+        // ELSE NULL
+        // END
+        c = new Case(EMPTY, Arrays.asList(
+            new IfConditional(EMPTY, new Equals(EMPTY, Literal.of(EMPTY, 1), Literal.of(EMPTY, 1)), Literal.NULL),
+            Literal.NULL));
+        assertEquals(DataType.NULL, c.dataType());
+
+        // CASE WHEN 1 = 1 THEN NULL
+        //      WHEN 2 = 2 THEN 'foo'
+        // ELSE NULL
+        // END
+        c = new Case(EMPTY, Arrays.asList(
+            new IfConditional(EMPTY, new Equals(EMPTY, Literal.of(EMPTY, 1), Literal.of(EMPTY, 1)), Literal.NULL),
+            new IfConditional(EMPTY, new Equals(EMPTY, Literal.of(EMPTY, 2), Literal.of(EMPTY, 2)), Literal.of(EMPTY, "foo")),
+            Literal.NULL));
+        assertEquals(DataType.KEYWORD, c.dataType());
+    }
+
     private List<Expression> mutateChildren(Case c) {
         boolean removeConditional = randomBoolean();
         List<Expression> expressions = new ArrayList<>(c.children().size());