Fix NullPointerException in HtmlRender when schema is null (#869)

* Initial plan

* Fix NullPointerException in HtmlRender when schema is null

Co-authored-by: DrSatyr <8143518+DrSatyr@users.noreply.github.com>

* Rename test methods and YAML files to include issue ID #865

Co-authored-by: DrSatyr <8143518+DrSatyr@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: DrSatyr <8143518+DrSatyr@users.noreply.github.com>

Author: Copilot

core/src/main/java/org/openapitools/openapidiff/core/output/HtmlRender.java

@@ -342,13 +342,15 @@ private LiTag li_missingRequest(String name, MediaType request) {
   }
 
   private LiTag li_changedRequest(String name, ChangedMediaType request) {
-    LiTag li =
-        li().with(div_changedSchema(request.getSchema()))
-            .withText(String.format("Changed body: '%s'", name));
-    if (request.isIncompatible() && !showAllChanges) {
-      incompatibilities(li, request.getSchema());
-    } else if (showAllChanges) {
-      allChanges(li, request.getSchema());
+    LiTag li = li().withText(String.format("Changed body: '%s'", name));
+    ChangedSchema schema = request.getSchema();
+    if (schema != null) {
+      li.with(div_changedSchema(schema));
+      if (request.isIncompatible() && !showAllChanges) {
+        incompatibilities(li, schema);
+      } else if (showAllChanges) {
+        allChanges(li, schema);
+      }
     }
     return li;
   }

core/src/test/java/org/openapitools/openapidiff/core/output/HtmlRenderTest.java

@@ -19,4 +19,28 @@ public void renderDoesNotFailWhenPropertyHasBeenRemoved() {
     render.render(diff, outputStreamWriter);
     assertThat(outputStream.toString()).isNotBlank();
   }
+
+  @Test
+  public void issue865_renderDoesNotFailWhenSchemaIsNullButExampleChanged() {
+    HtmlRender render = new HtmlRender();
+    ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
+    OutputStreamWriter outputStreamWriter = new OutputStreamWriter(outputStream);
+    ChangedOpenApi diff =
+        OpenApiCompare.fromLocations(
+            "issue-865-null-schema-1.yaml", "issue-865-null-schema-2.yaml");
+    render.render(diff, outputStreamWriter);
+    assertThat(outputStream.toString()).isNotBlank();
+  }
+
+  @Test
+  public void issue865_renderWithShowAllChangesDoesNotFailWhenSchemaIsNullButExampleChanged() {
+    HtmlRender render = new HtmlRender(true);
+    ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
+    OutputStreamWriter outputStreamWriter = new OutputStreamWriter(outputStream);
+    ChangedOpenApi diff =
+        OpenApiCompare.fromLocations(
+            "issue-865-null-schema-1.yaml", "issue-865-null-schema-2.yaml");
+    render.render(diff, outputStreamWriter);
+    assertThat(outputStream.toString()).isNotBlank();
+  }
 }

core/src/test/resources/issue-865-null-schema-1.yaml

@@ -0,0 +1,21 @@
+openapi: 3.0.2
+info:
+  description: Test for null schema issue
+  title: Null Schema Test
+  version: 1.0.0
+paths:
+  /test:
+    post:
+      requestBody:
+        content:
+          application/json:
+            example: original example
+            schema:
+              type: string
+      responses:
+        '200':
+          description: Success
+          content:
+            application/json:
+              schema:
+                type: string

core/src/test/resources/issue-865-null-schema-2.yaml

@@ -0,0 +1,21 @@
+openapi: 3.0.2
+info:
+  description: Test for null schema issue
+  title: Null Schema Test
+  version: 1.0.0
+paths:
+  /test:
+    post:
+      requestBody:
+        content:
+          application/json:
+            example: updated example
+            schema:
+              type: string
+      responses:
+        '200':
+          description: Success
+          content:
+            application/json:
+              schema:
+                type: string