fix handle missing yaml.ScalarNode (#129)

This bug was reported on https://github.com/go-gitea/gitea/issues/33657
Rewrite of (see below) was missing in this commit https://gitea.com/gsvd/act/commit/6cdf1e5788dbfa8fe9787a07178c28b64837a61f
```go
case string:
    acts[act] = []string{b}
```

Reviewed-on: https://gitea.com/gitea/act/pulls/129
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Guillaume S. <me@gsvd.dev>
Co-committed-by: Guillaume S. <me@gsvd.dev>

pkg/jobparser/model_test.go

@@ -58,6 +58,19 @@ func TestParseRawOn(t *testing.T) {
 				},
 			},
 		},
+		{
+			input: "on:\n  push:\n    branches: main",
+			result: []*Event{
+				{
+					Name: "push",
+					acts: map[string][]string{
+						"branches": {
+							"main",
+						},
+					},
+				},
+			},
+		},
 		{
 			input: "on:\n  branch_protection_rule:\n    types: [created, deleted]",
 			result: []*Event{