Support the comma as default string param separator

[wzhanw]

Which issue is resolved by this Pull Request:
Resolves #
We have a use case like that:
The iteration parameter string value is 1,2,3,4,5, but the separator is not set.
User wants the default separator should be a comma (,)

Description of your changes:
Try the default separator , in the last when interationParam string is not empty and contains the comma.

Environment tested:
Unit test passed.

• Python Version (use python --version):
• Tekton Version (use tkn version):
• Kubernetes Version (use kubectl version):
• OS (e.g. from /etc/os-release):

Checklist:

• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[wzhanw]

@Tomcli could you please review this PR? Thanks, this is a valid case on our customer side.

[wzhanw]

The ut failure was not caused by my code, it was caused by the cache store.

[Tomcli]

@wzhanw Although the caching code is returning error logs due to not running on Kubernetes, that didn't cause the unit test to fail. The only failed unit test is missing this error message.

kfp-tekton/tekton-catalog/pipeline-loops/pkg/reconciler/pipelinelooprun/pipelinelooprun_test.go

Lines 2074 to 2080 in 570c7de

	name: "iterate parameter not an array",
	pipelineloop: aPipelineLoop,
	run: runWithIterateParamNotAnArray,
	reason: pipelineloopv1alpha1.PipelineLoopRunReasonFailedValidation,
	wantEvents: []string{
	"Normal Started ",
	`Warning Failed Cannot determine number of iterations: The value of the iterate parameter "current-item" can not transfer to array`,

[Tomcli]

@wzhanw Thanks for the PR. Your use case make sense. Once the unit tests are fixed, I can merge this PR

[wzhanw]

@Tomcli thanks, let me fix the failure in the unit test.

[wzhanw]

@Tomcli please review again, I fixed the unit test failure.

[Tomcli]

thanks @wzhanw
/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Tomcli, wzhanw

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