allow null values for extensions properties in openapi object

modules/swagger-core/src/main/java/io/swagger/v3/core/jackson/mixin/OpenAPIMixin.java

@@ -3,6 +3,7 @@
 import com.fasterxml.jackson.annotation.JsonAnyGetter;
 import com.fasterxml.jackson.annotation.JsonAnySetter;
 import com.fasterxml.jackson.annotation.JsonIgnore;
+import com.fasterxml.jackson.annotation.JsonInclude;
 import com.fasterxml.jackson.databind.annotation.JsonSerialize;
 import io.swagger.v3.core.jackson.PathsSerializer;
 import io.swagger.v3.oas.models.PathItem;
@@ -12,6 +13,7 @@
 public abstract class OpenAPIMixin {
 
     @JsonAnyGetter
+    @JsonInclude
     public abstract Map<String, Object> getExtensions();
 
     @JsonAnySetter

modules/swagger-core/src/test/java/io/swagger/v3/core/serialization/JsonSerializationTest.java

@@ -18,6 +18,9 @@
 import org.testng.annotations.Test;
 import org.yaml.snakeyaml.LoaderOptions;
 
+import java.util.HashMap;
+import java.util.Map;
+
 import static org.testng.Assert.assertEquals;
 import static org.testng.Assert.assertFalse;
 
@@ -55,6 +58,19 @@ public void testExtension() throws Exception {
 
     }
 
+    @Test
+    public void testExtensionObjectWithProperties() throws Exception {
+        final Map<String, Object> extensionObjectProps = new HashMap<>();
+        extensionObjectProps.put("x-foo-bar", "foo bar");
+        extensionObjectProps.put("x-bar-foo", null);
+
+        OpenAPI swagger = new OpenAPI();
+        swagger.addExtension("x-extension-with-properties", extensionObjectProps);
+
+        String swaggerJson = Json.mapper().writeValueAsString(swagger);
+        assertEquals(swaggerJson, "{\"openapi\":\"3.0.1\",\"x-extension-with-properties\":{\"x-foo-bar\":\"foo bar\",\"x-bar-foo\":null}}");
+    }
+
     @Test
     public void testSerializeASpecWithResponseReferences() throws Exception {
         OpenAPI swagger = new OpenAPI()