Fix NullPointerException in MarkdownRender when schema is null (#871)

* Initial plan

* Fix NullPointerException in MarkdownRender, AsciidocRender, and ConsoleRender when schema is null

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

* Add issue ID prefix to test method names for issue #870

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/AsciidocRender.java

@@ -182,7 +182,7 @@ private String itemContent(
         .append(itemContent("Schema:", "", indent))
         .append(changedMediaType.isCompatible() ? "Backward compatible" : "Broken compatibility")
         .append(System.lineSeparator());
-    if (!changedMediaType.isCompatible()) {
+    if (!changedMediaType.isCompatible() && changedMediaType.getSchema() != null) {
       sb.append(incompatibilities(changedMediaType.getSchema()));
     }
     return sb.toString();

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

@@ -171,7 +171,7 @@ private String itemContent(
         .append("Schema: ")
         .append(changedMediaType.isCompatible() ? "Backward compatible" : "Broken compatibility")
         .append(System.lineSeparator());
-    if (!changedMediaType.isCompatible()) {
+    if (!changedMediaType.isCompatible() && changedMediaType.getSchema() != null) {
       sb.append(incompatibilities(changedMediaType.getSchema()));
     }
     return sb.toString();

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

@@ -274,7 +274,11 @@ protected String itemContent(String title, String mediaType, MediaType content)
   }
 
   protected String itemContent(int deepness, String mediaType, ChangedMediaType content) {
-    return itemContent("Changed content type", mediaType) + schema(deepness, content.getSchema());
+    String result = itemContent("Changed content type", mediaType);
+    if (content.getSchema() != null) {
+      result += schema(deepness, content.getSchema());
+    }
+    return result;
   }
 
   protected String schema(ChangedSchema schema) {

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

@@ -93,4 +93,16 @@ public void validateAsciiDocRangeStatus() {
                 + "\n"
                 + "WARNING: API changes broke backward compatibility\n");
   }
+
+  @Test
+  public void issue870_renderDoesNotFailWhenSchemaIsNullButExampleChanged() {
+    AsciidocRender render = new AsciidocRender();
+    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/java/org/openapitools/openapidiff/core/output/ConsoleRenderTest.java

@@ -76,4 +76,16 @@ public void renderShowsWhatsChangedSectionWithCorrectFormattingWhenEndpointIsCha
         .contains("What's Changed")
         .containsSubsequence("- GET    /widgets", "Parameter:", "- Changed query-param-1 in query");
   }
+
+  @Test
+  public void issue870_renderDoesNotFailWhenSchemaIsNullButExampleChanged() {
+    ConsoleRender render = new ConsoleRender();
+    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/java/org/openapitools/openapidiff/core/output/MarkdownRenderTest.java

@@ -40,4 +40,16 @@ public void renderDoesNotFailWhenHTTPStatusCodeIsRange() {
     render.render(diff, outputStreamWriter);
     assertThat(outputStream.toString()).isNotBlank();
   }
+
+  @Test
+  public void issue870_renderDoesNotFailWhenSchemaIsNullButExampleChanged() {
+    MarkdownRender render = new MarkdownRender();
+    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();
+  }
 }