Rewrite tests so AutoValue classes are always abstract, even if they have no properties.

PiperOrigin-RevId: 751056240

Author: eamonnmcmanus

common/src/main/java/dev/cel/common/types/UnspecifiedType.java

@@ -32,7 +32,7 @@
 @CheckReturnValue
 @Immutable
 @Internal
-public class UnspecifiedType extends CelType {
+public abstract class UnspecifiedType extends CelType {
 
   @Override
   public CelKind kind() {