correct handling of empty request body with Handlebars (wiremock#2546) (

wiremock#2552)

Instead of having `jsonpath` in handlebars throwing an `IllegalArgumentException`,
empty bodies are handled the same way as `null` bodies: as empty string

src/main/java/com/github/tomakehurst/wiremock/extension/responsetemplating/helpers/HandlebarsJsonPathHelper.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2017-2023 Thomas Akehurst
+ * Copyright (C) 2017-2024 Thomas Akehurst
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -37,7 +37,7 @@ public class HandlebarsJsonPathHelper extends HandlebarsHelper<Object> {
 
   @Override
   public Object apply(final Object input, final Options options) throws IOException {
-    if (input == null) {
+    if (input == null || (input instanceof String && ((String) input).isEmpty())) {
       return "";
     }
 

src/test/java/com/github/tomakehurst/wiremock/extension/responsetemplating/helpers/HandlebarsJsonPathHelperTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2017-2023 Thomas Akehurst
+ * Copyright (C) 2017-2024 Thomas Akehurst
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -62,7 +62,7 @@ public void mergesASimpleValueFromRequestIntoResponseBody() {
   }
 
   @Test
-  public void incluesAnErrorInTheResponseBodyWhenTheJsonPathIsInvalid() {
+  public void includesAnErrorInTheResponseBodyWhenTheJsonPathIsInvalid() {
     final ResponseDefinition responseDefinition =
         transform(
             transformer,
@@ -264,6 +264,11 @@ public void rendersAnEmptyStringWhenJsonIsNull() {
     testHelperError(helper, null, "$.test", is(""));
   }
 
+  @Test
+  public void rendersAnEmptyStringWhenJsonIsEmptyString() {
+    testHelperError(helper, "", "$.test", is(""));
+  }
+
   @Test
   public void rendersAMeaningfulErrorWhenJsonPathIsNull() {
     testHelperError(