Task create should not attempt git webhooks when git-token isn't provided

[ehotinger]

FEATURE REQUEST:
When using az acr build . we ignore push if -t is omitted.
Similarly, when creating a task, we could follow the same convention.

What happened?:

az acr build-task create \
  -n demo42web \
  --context https://github.com/demo42/web \
  -t demo42/web:{{.Build.ID}} \
  -f ./src/WebUI/Dockerfile \
  --registry $ACR_NAME 

Running the above, without the --git-access-token causes a failure
To fix this, i need to provide --commit-trigger-enabled

What did you expect to happen?:
Not specifying a token would not attempt to create the webhook and be equivalent to --commit-trigger-enabled false

Original issue: Azure/acr-builder#280