couler.set_dependencies do not set dependencies correctly

[zhu jianjiang]

Summary

when set_dependencies None dep, have no deal with this issue
minimal Reproducible code：

        couler.set_dependencies(
            lambda: job_a(message="A"), dependencies=None
        )
        couler.set_dependencies(
            lambda: job_b(message="B"), dependencies="A.Succeeded"
        )
        couler.set_dependencies(
            lambda: job_c(message="C"), dependencies=None
        )

What happened/what you expected to happen?

        wf_tree = couler.workflow_yaml()
        tasks = wf_tree["spec"]["templates"][0]["dag"]["tasks"]
        deps = tasks[2].get("dependencies")

Before my pr, deps will be B ,but it should be None

Diagnostics

What is the version of Couler you are using?
master branch

I'll fix it
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