skip the jobs that have no tasks during the close session step in gang plugin

[Wang-Kai]

What type of PR is this?

/kind flake
/area scheduling

What this PR does / why we need it:

In the OnSessionClose function, the new PR skip the unavailable job. Unnecessary podgroup condition updates are avoided in the following scenarios:

1. In the vj deletion scenario, the Pod deletion event is watched earlier than the PG deletion event. At this time, len(jobInfo.Tasks) == 0 && jobInfo.PodGroup != nil, and a new PodGroupUnschedulableType condition is added
2. In the vj creation scenario, the podgroup has been created but the pod has not been created yet. In this case, a new PodGroupUnschedulableType condition is added similar to scenario 1

Which issue(s) this PR fixes:

Fixes #4011

Special notes for your reviewer:

NONE

Does this PR introduce a user-facing change?

NONE

[hwdef]

/lgtm
/ok-to-test

[Monokaix]

/approve

[volcano-sh-bot]

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