Fix null body response to empty in ApiClient (#579)

## Changes
<!-- Summary of your changes that are easy to understand -->
Some delete end points return null instead of empty body, we fix that in
SDK Client since the APIs are not available publicly and support isn't
available for them yet.

## Tests
<!-- 
How is this tested? Please see the checklist below and also describe any
other relevant tests
-->
Thanks to @sodle-splunk for contributing the unit test
(#641)
- [x] `make test` run locally
- [x] `make fmt` applied
- [ ] relevant integration tests applied

---------

Co-authored-by: Scott Odle <sodle@splunk.com>

databricks/sdk/core.py

@@ -145,11 +145,14 @@ def do(self,
         if not len(response.content):
             return resp
 
-        json = response.json()
-        if isinstance(json, list):
-            return json
+        jsonResponse = response.json()
+        if jsonResponse is None:
+            return resp
+
+        if isinstance(jsonResponse, list):
+            return jsonResponse
 
-        return {**resp, **json}
+        return {**resp, **jsonResponse}
 
     @staticmethod
     def _is_retryable(err: BaseException) -> Optional[str]:

tests/test_core.py

@@ -335,6 +335,21 @@ def test_shares(config, requests_mock):
     assert requests_mock.last_request.json() == {'changes': [{'principal': 'principal'}]}
 
 
+def test_deletes(config, requests_mock):
+    requests_mock.delete("http://localhost/api/2.0/preview/sql/alerts/alertid",
+                         request_headers={"User-Agent": config.user_agent},
+                         text="null",
+                         )
+
+    w = WorkspaceClient(config=config)
+    res = w.alerts.delete(alert_id="alertId")
+
+    assert requests_mock.call_count == 1
+    assert requests_mock.called
+
+    assert res is None
+
+
 def test_error(config, requests_mock):
     errorJson = {
         "message":