fix(output): check if ChangedContent is null or not

Author: quen2404

src/main/java/com/qdesrame/openapi/diff/output/MarkdownRender.java

@@ -212,6 +212,9 @@ protected String itemHeader(String title, String mediaType, String description)
   }
 
   protected String bodyContent(String prefix, ChangedContent changedContent) {
+    if (changedContent == null) {
+      return "";
+    }
     StringBuilder sb = new StringBuilder("\n");
     sb.append(listContent(prefix, "New content type", changedContent.getIncreased()));
     sb.append(listContent(prefix, "Deleted content type", changedContent.getMissing()));