Fix an invalid signature (#33984)

This code is creating a field with the same type as a method return
type. The return type is erased, but calling `getGenericSignature()`
turns the erased type back into a generic type. For methods with type
parameters, this results in signatures that are invalid for field
declarations, for example
`<T:Ljava/lang/Object;>Ljava/lang/Object;Ljava/io/Serializable;`. Future
versions of bytebuddy reject the invalid signature.

See also raphw/byte-buddy#1771

sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/reflect/ByteBuddyDoFnInvokerFactory.java

@@ -1311,7 +1311,7 @@ public StackManipulation.Size apply(MethodVisitor mv, Context context) {
         mv.visitLocalVariable(
             "res",
             returnType.getDescriptor(),
-            returnType.getGenericSignature(),
+            null,
             wrapStart,
             wrapEnd,
             returnVarIndex);