[AIRFLOW-5705] Fix bugs in AWS SSM Secrets Backend

[kaxil]

#6376 had some bugs that this PR fixes

• AwsSsmSecretsBackend.get_conn_uri should return None if parameter does not exist
• This PR adds error handling if Parameter is not Found
• Adds tests case and fixes current tests

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Issue link: AIRFLOW-5705

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cc @dstandish

[codecov-io]

Codecov Report

Merging #7745 into master will decrease coverage by 22.35%.
The diff coverage is 87.5%.

@@             Coverage Diff             @@
##           master    #7745       +/-   ##
===========================================
- Coverage   86.94%   64.59%   -22.36%     
===========================================
  Files         915      914        -1     
  Lines       44176    44171        -5     
===========================================
- Hits        38411    28533     -9878     
- Misses       5765    15638     +9873

Impacted Files	Coverage Δ
airflow/secrets/__init__.py	94.87% <100%> (-5.13%)	⬇️
airflow/providers/amazon/aws/secrets/ssm.py	94.28% <85.71%> (+27.61%)	⬆️
...low/contrib/operators/wasb_delete_blob_operator.py	0% <0%> (-100%)	⬇️
airflow/contrib/hooks/vertica_hook.py	0% <0%> (-100%)	⬇️
airflow/contrib/sensors/__init__.py	0% <0%> (-100%)	⬇️
airflow/hooks/mssql_hook.py	0% <0%> (-100%)	⬇️
...viders/docker/example_dags/example_docker_swarm.py	0% <0%> (-100%)	⬇️
airflow/hooks/webhdfs_hook.py	0% <0%> (-100%)	⬇️
airflow/contrib/sensors/emr_base_sensor.py	0% <0%> (-100%)	⬇️
...irflow/contrib/operators/slack_webhook_operator.py	0% <0%> (-100%)	⬇️
... and 494 more

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