fix(actions): validate workflow param to prevent 500 error (#37546) (#37554)

Backport #37546 by @KalashThakare

This PR fixes issue #37523:

1. Prevents a 500 error on the Actions page when disabling workflows
with an empty workflow parameter
2. Uses a single **ctx.JSONError** in the handler to return 400 Bad
Request with the message “workflow is required” for empty input

Co-authored-by: Kalash Thakare ☯︎ <kalashthakare898@gmail.com>

routers/web/repo/actions/view.go

@@ -949,7 +949,7 @@ func EnableWorkflowFile(ctx *context_module.Context) {
 func disableOrEnableWorkflowFile(ctx *context_module.Context, isEnable bool) {
 	workflow := ctx.FormString("workflow")
 	if len(workflow) == 0 {
-		ctx.ServerError("workflow", nil)
+		ctx.JSONError("workflow is required")
 		return
 	}