[patch] Return none when there is no subscription

[durera]

When there is no ibm-mas subscription for the MAS instance ID provided return None, resolve this failure seen when trying to upgrade an instance that does not exist:

IBM Maximo Application Suite Admin CLI v12.1.1-pre.master
Powered by https://github.com/ibm-mas/ansible-devops/ and https://tekton.dev/
Traceback (most recent call last):
  File "/opt/app-root/bin/mas-cli", line 65, in <module>
    app.upgrade(argv[2:])
  File "/opt/app-root/lib64/python3.9/site-packages/mas/cli/upgrade/app.py", line 73, in upgrade
    current_mas_channel = getMasChannel(self.dynamicClient, instanceId)
  File "/opt/app-root/lib64/python3.9/site-packages/mas/devops/mas.py", line 154, in getMasChannel
    return masSubscription.spec.channel
AttributeError: 'NoneType' object has no attribute 'spec'