fix: avoid setting limit unless set in values

[jawnsy]

The default value for --limit configured in our typer.Option is None; however, users cannot explicitly set the value to None: tiangolo/typer#530

Closes: #192

[jawnsy]

Testing this with a Prefect server and worker in the same environment:

$ helm upgrade --install --reset-values --set=postgresql.enabled=false --set=postgresql.useSubChart=false prefect-ser
ver ./charts/prefect-server

$ helm upgrade --install --reset-values --set=worker.apiConfig=server --set=worker.serverApiConfi.apiUrl=http://prefect-server.jawnsy-test.svc.cluster.local:4200/api --set=worker.config.workPool=test prefect-worker ./charts/prefect-worker/
Release "prefect-worker" has been upgraded. Happy Helming!
NAME: prefect-worker
LAST DEPLOYED: Mon Jul  3 09:07:02 2023
NAMESPACE: jawnsy-test
STATUS: deployed
REVISION: 4
TEST SUITE: None
NOTES:
1. Check Prefect worker connections in the prefect UI at "http://127.0.0.1:4200/api"

$ k logs prefect-worker-7448575857-dbvdc
Worker 'KubernetesWorker b38e1d41-2945-46d4-9559-104b59417675' started!

$ helm upgrade --install --reset-values --set=worker.apiConfig=server --set=worker.serv
erApiConfig.apiUrl=http://prefect-server.jawnsy-test.svc.cluster.local:4200/api --set=worker.config.workPool=test --set=worker.config.limit=100 prefect-worker ./charts/prefect-worker/
Release "prefect-worker" has been upgraded. Happy Helming!
NAME: prefect-worker
LAST DEPLOYED: Mon Jul  3 09:36:40 2023
NAMESPACE: jawnsy-test
STATUS: deployed
REVISION: 9
TEST SUITE: None
NOTES:
1. Check Prefect worker connections in the prefect UI at "http://prefect-server.jawnsy-test.svc.cluster.local:4200/api"

$ k logs prefect-worker-6544fd7bbc-2x7q7
Worker 'KubernetesWorker a4204f66-8519-4d88-a58f-ca30553a973f' started!