fix(actions): prevent panic when workflow contains null jobs (#37570) (#37576)

Backport #37570 by @Exgene

## 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: Kausthubh J Rao <105716675+Exgene@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/actions/jobparser/jobparser_test.go

@@ -59,6 +59,13 @@ func TestParse(t *testing.T) {
 			wantErr: false,
 		},
 	}
+	invalidFileTests := []struct {
+		name string
+	}{
+		{name: "null_job_implicit"},
+		{name: "null_job_explicit"},
+	}
+
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			content := ReadTestdata(t, tt.name+".in.yaml")
@@ -84,4 +91,14 @@ func TestParse(t *testing.T) {
 			assert.Equal(t, string(want), builder.String())
 		})
 	}
+
+	for _, tt := range invalidFileTests {
+		t.Run(tt.name, func(t *testing.T) {
+			content := ReadTestdata(t, tt.name+".in.yaml")
+			require.NotPanics(t, func() {
+				_, err := Parse(content)
+				require.Error(t, err)
+			})
+		})
+	}
 }