fix: add missing @nullable annotations in javaagent-tooling and muzzle

- VirtualFieldChecker.getAnnotationNode(): missing @nullable on return
- HelperInjector: @nullable on helpersSource (first constructor),
  instrumentation (forDynamicTypes), and classLoader (transform)
Signed-off-by: Gregor Zeitlinger <gregor.zeitlinger@grafana.com>

Author: zeitlinger

javaagent-tooling/src/main/java/io/opentelemetry/javaagent/tooling/instrumentation/indy/VirtualFieldChecker.java

@@ -7,6 +7,7 @@
 
 import io.opentelemetry.instrumentation.api.util.VirtualField;
 import io.opentelemetry.javaagent.extension.instrumentation.internal.AsmApi;
+import javax.annotation.Nullable;
 import net.bytebuddy.asm.Advice;
 import org.objectweb.asm.ClassReader;
 import org.objectweb.asm.MethodVisitor;
@@ -61,6 +62,7 @@ public void visitMethodInsn(
         });
   }
 
+  @Nullable
   private static AnnotationNode getAnnotationNode(MethodNode source, Type type) {
     if (source.visibleAnnotations != null) {
       for (AnnotationNode annotationNode : source.visibleAnnotations) {

muzzle/src/main/java/io/opentelemetry/javaagent/tooling/HelperInjector.java

@@ -124,7 +124,7 @@ public HelperInjector(
       String requestingName,
       List<String> helperClassNames,
       List<HelperResource> helperResources,
-      ClassLoader helpersSource,
+      @Nullable ClassLoader helpersSource,
       @Nullable Instrumentation instrumentation) {
     this.requestingName = requestingName;