Handle null class body in KotlinPrinter (#6805)

KotlinPrinter.visitClassDeclaration0 crashed with a NullPointerException
when printing a class declaration with a null body. This happens when
BlockStatementTemplateGenerator builds context for a context-sensitive
template: it creates a stripped-down class via withBody(null) and prints
it to generate the template context. The Java printer handles this fine
(visit() is null-safe), but the Kotlin printer accessed getBody()
without a null check.

Author: Jenson3210

rewrite-kotlin/src/main/java/org/openrewrite/kotlin/internal/KotlinPrinter.java

@@ -728,7 +728,7 @@ private J.ClassDeclaration visitClassDeclaration0(J.ClassDeclaration classDecl,
                 visitContainer("where", typeConstraints.getPadding().getConstraints(), JContainer.Location.TYPE_PARAMETERS, ",", "", p);
             }
 
-            if (!classDecl.getBody().getMarkers().findFirst(OmitBraces.class).isPresent()) {
+            if (classDecl.getBody() != null && !classDecl.getBody().getMarkers().findFirst(OmitBraces.class).isPresent()) {
                 visit(classDecl.getBody(), p);
             }
             afterSyntax(classDecl, p);

rewrite-kotlin/src/test/java/org/openrewrite/kotlin/KotlinTemplateTest.java

@@ -22,6 +22,10 @@
 import org.openrewrite.java.tree.J;
 import org.openrewrite.test.RewriteTest;
 
+import org.openrewrite.java.JavaTemplate;
+import org.openrewrite.java.tree.Statement;
+import org.openrewrite.test.TypeValidation;
+
 import java.net.URISyntaxException;
 import java.nio.file.Path;
 import java.util.List;
@@ -61,6 +65,45 @@ fun foo() {
           ));
     }
 
+    @Test
+    void addStatementToMethodInClass() {
+        rewriteRun(
+          spec -> spec.typeValidationOptions(TypeValidation.none())
+            .recipe(toRecipe(() -> new KotlinVisitor<>() {
+              @Override
+              public J visitMethodDeclaration(J.MethodDeclaration method, ExecutionContext ctx) {
+                  J.MethodDeclaration m = (J.MethodDeclaration) super.visitMethodDeclaration(method, ctx);
+                  if (m.getSimpleName().equals("configure")) {
+                      List<Statement> statements = m.getBody().getStatements();
+                      if (statements.stream().noneMatch(s -> s.toString().contains("println"))) {
+                          return JavaTemplate.builder("println(\"added\")")
+                            .contextSensitive()
+                            .build()
+                            .apply(getCursor(), statements.get(statements.size() - 1).getCoordinates().after());
+                      }
+                  }
+                  return m;
+              }
+          })),
+          kotlin(
+            """
+              class MyConfig {
+                  fun configure(value: Int) {
+                      val x = value + 1
+                  }
+              }
+              """,
+            """
+              class MyConfig {
+                  fun configure(value: Int) {
+                      val x = value + 1
+                      println("added")
+                  }
+              }
+              """
+          ));
+    }
+
     @Test
     void parserClasspath() {
         rewriteRun(