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/testdata/null_job_explicit.in.yaml

@@ -0,0 +1,9 @@
+# null_job_explicit.in.yaml
+on: push
+jobs:
+  empty: null
+  notempty:
+    needs: empty
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo ok

modules/actions/jobparser/testdata/null_job_implicit.in.yaml

@@ -0,0 +1,9 @@
+# null_job_implicit.in.yaml
+on: push
+jobs:
+  empty:
+  notempty:
+    needs: empty
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo ok