fix: continue_on_success_ratio without slice raise error

[saltball]

For some conditions, the slice expression do not worked for if_expression when combining and split the job groups. This PR tries to fix this.
The problem hardly effect the normal users especially the executor users. I'll check it further and provide details later.

[zjgemi]

Can you provide an example?
It is known that for some certain Argo versions, the if_expression for group_size will cause error. I'm not sure if the issue is related to that.