Add a default constructor for test only to pass testing in Java recipe guide.

[addozhang]

What's changed?

java.lang.AssertionError: [Recipe must be able to instantiate via RecipeIntrospectionUtils] 
Expecting code not to raise a throwable but caught
  "org.openrewrite.config.RecipeIntrospectionException: Unable to call primary constructor for Recipe class ...
...

I found this is issue with this commit. In this commit, The RewriteTest will invoke RecipeIntrospectionUtils.constructRecipe(recipe.getClass()) to do some validation. This requires our recipe should contain a default constructor.

Add a default constructor will fix it.

    @TestOnly
    public SayHelloRecipe() {
        fullyQualifiedClassName = "";
    }    

Have you considered any alternatives or workarounds?

Not sure.

Any additional context

Checklist

• I've added unit tests to cover both positive and negative cases
• I've read and applied the recipe conventions and best practices
• I've used the IntelliJ IDEA auto-formatter on affected files

[timtebeek]

Thanks a lot for the fix here @addozhang , also for pulling in that context of where it now fails. I've removed the @TestOnly annotation as we want to minimize what folks need to have available, as we similarly stripped Lombok from the test examples as well. With that we're good to merge. ❤️