Fix deserialisation of empty body in HTTP responses (#426)

## What changes are proposed in this pull request?
Fix deserialisation of empty body in HTTP responses

## How is this tested?
Added unit test. The unit test fails without this fix.

Author: hectorcast-db

NEXT_CHANGELOG.md

@@ -5,6 +5,7 @@
 ### New Features and Improvements
 
 ### Bug Fixes
+* Fix issue deserializing HTTP responses with an empty body ([#426](https://github.com/databricks/databricks-sdk-java/pull/426)).
 
 ### Documentation
 

databricks-sdk-java/src/main/java/com/databricks/sdk/core/ApiClient.java

@@ -421,7 +421,8 @@ public <T> void deserialize(Response response, T object) throws IOException {
           field.setAccessible(false);
         }
       }
-    } else if (response.getBody() != null) {
+      // mapper does support empty JSON "{}", but not empty body ""
+    } else if (response.getBody() != null && response.getBody().available() != 0) {
       mapper.readerForUpdating(object).readValue(response.getBody());
     }
   }

databricks-sdk-java/src/test/java/com/databricks/sdk/core/ApiClientTest.java

@@ -268,6 +268,17 @@ void failIdempotentRequestAfterTooManyRetries() throws IOException {
         "Request GET /api/2.0/sql/sessions/ failed after 4 retries");
   }
 
+  @Test
+  void testEmptyBody() throws IOException {
+    MyEndpointResponse response = new MyEndpointResponse();
+    Request request = getBasicRequest();
+    Response rawResponse = new Response(request, 200, "OK", Collections.emptyMap(), "");
+    ApiClient client =
+        getApiClient(request, Collections.singletonList(new SuccessfulResponse(rawResponse)));
+
+    client.deserialize(rawResponse, response);
+  }
+
   @Test
   void retryDatabricksApi12RetriableError() throws IOException {
     Request req = getBasicRequest();