fix(actions): prevent panic when workflow contains null jobs (#37570)

## The issue

Closes #37568. Basically due to empty fields being present in the
actions file, the jobs would be produced as `nil` inside `jobparser.go`
. Because of this when we call `Parse` on the `jobparser` module.

```go
Needs:   job.Needs(),
```

would propagate the `nil` job down the chain. 

## The fix

For now i decide to fix it by guarding with an `if job == nil` check.

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

modules/actions/jobparser/jobparser.go

@@ -31,6 +31,9 @@ func Parse(content []byte, options ...ParseOption) ([]*SingleWorkflow, error) {
 	}
 	results := map[string]*JobResult{}
 	for id, job := range origin.Jobs {
+		if job == nil {
+			return nil, fmt.Errorf("needed job not found: %q", id)
+		}
 		results[id] = &JobResult{
 			Needs:   job.Needs(),
 			Result:  pc.jobResults[id],