Optimize annotations without object bodies (#1843)

This skips some unnecessary steps when evaluating annotations that
have no attached object bodies by returning an EmptyObjectLiteralNode
instead of a PropertiesLiteralNode

pkl-core/src/main/java/org/pkl/core/ast/builder/AstBuilder.java

@@ -2224,17 +2224,7 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
 
     var bodyCtx = annotation.getBody();
     if (bodyCtx == null) {
-      var currentScope = symbolTable.getCurrentScope();
-      //noinspection ConstantConditions
-      return PropertiesLiteralNodeGen.create(
-          createSourceSection(annotation),
-          language,
-          currentScope.getQualifiedName(),
-          currentScope.isCustomThisScope(),
-          null,
-          new UnresolvedTypeNode[0],
-          EconomicMaps.create(),
-          verifyNode);
+      return EmptyObjectLiteralNodeGen.create(createSourceSection(annotation), verifyNode);
     }
 
     return symbolTable.enterAnnotationScope(

pkl-core/src/main/java/org/pkl/core/ast/expression/literal/CheckIsAnnotationClassNode.java

@@ -24,7 +24,7 @@ import org.pkl.core.ast.type.UnresolvedTypeNode;
 import org.pkl.core.runtime.*;
 
 public final class CheckIsAnnotationClassNode extends ExpressionNode {
-  @Child private UnresolvedTypeNode unresolvedTypeNode;
+  @Child private @Nullable UnresolvedTypeNode unresolvedTypeNode;
   @Child private @Nullable TypeNode typeNode;
 
   public CheckIsAnnotationClassNode(UnresolvedTypeNode unresolvedTypeNode) {
@@ -37,6 +37,7 @@ public final class CheckIsAnnotationClassNode extends ExpressionNode {
     if (typeNode == null) {
       // invalidation is done by insert()
       CompilerDirectives.transferToInterpreter();
+      assert unresolvedTypeNode != null;
       typeNode = insert(unresolvedTypeNode.execute(frame));
       unresolvedTypeNode = null;
     }