[AppService] Bugfix: Handle if only single publish profile is returned

[calvinsID]

Description
In some apps with only single publish profile, API doesn't return a list andaz webapp deployment source config-zip breaks

Testing Guide
Run az webapp deployment source config-zip on a webapp that only has a single publish profile, the command will error out.
The behaviour will not be changed if the webapp has multiple publish profiles

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[panchagnula]

LGTM - do we need to re-record any tests?

[qwordy]

@calvinsID

ERROR: /home/vsts/work/1/s/src/azure-cli/azure/cli/command_modules/appservice/custom.py:1059:5: F841 local variable 'e' is assigned to but never used
/home/vsts/work/1/s/src/azure-cli/azure/cli/command_modules/appservice/custom.py:1059:5: F841 local variable 'e' is assigned to but never used
2     F841 local variable 'e' is assigned to but never used

ERROR: Flake8: FAILED

ERROR: ************* Module azure.cli.command_modules.appservice.custom
src/azure-cli/azure/cli/command_modules/appservice/custom.py:1059:4: W0612: Unused variable 'e' (unused-variable)
src/azure-cli/azure/cli/command_modules/appservice/custom.py:1059:4: W0612: Unused variable 'e' (unused-variable)