fix: resolve NullAway violations after rebase onto main

- AgentStarterImpl: add @nullable to two ClassFileTransformer.transform
  methods that can return null, matching the existing pattern on the
  sibling LaunchHelperClassFileTransformer.
- HelperInjector: guard null helpersSource with requireNonNull at the
  call into HelperClassDefinition.create, which delegates to
  BytecodeWithUrl.create (rejects null with IllegalArgumentException).
  The message documents the runtime contract: null helpersSource is
  only valid when helperClassNames is empty.
Signed-off-by: Gregor Zeitlinger <gregor.zeitlinger@grafana.com>

Author: zeitlinger

javaagent-tooling/src/main/java/io/opentelemetry/javaagent/tooling/AgentStarterImpl.java

@@ -261,6 +261,7 @@ public void visitMethodInsn(
 
   private static class AgentBuilderDefaultClassFileTransformer implements ClassFileTransformer {
 
+    @Nullable
     @Override
     public byte[] transform(
         ClassLoader loader,
@@ -295,6 +296,7 @@ public FieldVisitor visitField(
 
   private static class CallbackRegistrationClassFileTransformer implements ClassFileTransformer {
 
+    @Nullable
     @Override
     public byte[] transform(
         ClassLoader loader,

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

@@ -134,7 +134,11 @@ public HelperInjector(
             .map(
                 className ->
                     HelperClassDefinition.create(
-                        className, helpersSource, InjectionMode.CLASS_ONLY))
+                        className,
+                        requireNonNull(
+                            helpersSource,
+                            "helpersSource must not be null when helperClassNames is non-empty"),
+                        InjectionMode.CLASS_ONLY))
             .collect(toList());
 
     this.helperClassesGenerator = (cl) -> helpers;