SDK - Fixed the non-string items in the command-line arguments (#1197)
SDK - Fixed the non-string items in the command-line arguments (#1197)

* SDK - Fixed the non-string items if the command-line arguments
Fixes a bug discovered by Makoto.
When you pass non-string arguments to the ContainerOp.command, they're passed as such to the resulting Argo workflow which then cannot be loaded in back-end due to the item types.
This PR ensures the items are converted to string.

* Removed the if_none parameter

It wasn't used anywhere.

* Specified that the argument is optional.

kubeflow/pipelines,kubeflow/pipelines,kubeflow/pipelines,kubeflow/pipelines,kubeflow/pipelines,kubeflow/pipelines