[AIRFLOW-6385] BugFix: SlackAPIPostOperator fails when blocks not set

[kaxil]

Issue:
After upgrade to 1.10.7 all slack notifications became faulty
SlackAPIPostOperator fails when blocks not set

    t_slack_ok = SlackAPIPostOperator(
        task_id = 'notify_ok',
        token = Variable.get('slack.token'),
        channel = Variable.get('slack.channel'),
        text = ':rocket: {{ dag.dag_id }} for {{ ds }} has succeeded',
        dag = dag)

Causing an operator error:

[2019-12-28 21:55:14,668] {taskinstance.py:1088} ERROR - Slack API call failed (invalid_blocks_format)
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/airflow/models/taskinstance.py", line 955, in _run_raw_task
    result = task_copy.execute(context=context)
  File "/usr/local/lib/python3.6/dist-packages/airflow/operators/slack_operator.py", line 85, in execute
    slack.call(self.method, self.api_params)
  File "/usr/local/lib/python3.6/dist-packages/airflow/hooks/slack_hook.py", line 62, in call
    raise AirflowException(msg)

Fix:
It is because the default value is None and json.dumps(None) is null which is not valid for Slack message layout: https://api.slack.com/messaging/composing/layouts

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Link to JIRA issue: https://issues.apache.org/jira/browse/AIRFLOW-6385

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(*) For document-only changes, no JIRA issue is needed. Commit message starts [AIRFLOW-XXXX].

---

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[codecov-io]

Codecov Report

Merging #7022 into master will decrease coverage by 0.23%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #7022      +/-   ##
==========================================
- Coverage   84.73%   84.49%   -0.24%     
==========================================
  Files         679      679              
  Lines       38598    38598              
==========================================
- Hits        32706    32614      -92     
- Misses       5892     5984      +92

Impacted Files	Coverage Δ
airflow/operators/slack_operator.py	100% <100%> (ø)	⬆️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/executors/sequential_executor.py	56% <0%> (-32%)	⬇️
airflow/kubernetes/refresh_config.py	50.98% <0%> (-23.53%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	78.75% <0%> (-20%)	⬇️
airflow/utils/helpers.py	74.5% <0%> (-7.19%)	⬇️
airflow/utils/dag_processing.py	80.95% <0%> (-7.05%)	⬇️
airflow/jobs/scheduler_job.py	88.1% <0%> (-1.18%)	⬇️
... and 6 more

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