fix(backend): fix skip check crash when flow-level skip_expr triggers on first module with skip_if (#8111)

When a flow has a flow-level `skip_expr` (or `no_flow_overlap`) and the first
module has `skip_if` defined, the flow-level condition returns `UpdateFlow`
before any identity job is created. The `UpdateFlow` path passes `Uuid::nil()`
as `job_id_for_status`, causing `fetch_one` to fail with "no rows returned".

- Change `fetch_one` to `fetch_optional` so a missing row returns false
- Short-circuit the DB query with `stop_early && skip_if_stop_early` so both
  skip mechanisms (identity job check and early-stop skip flag) are considered
- Also fixes the logical gap where a module with both `skip_if` and
  `stop_after_if` would only check the identity job, ignoring the early-stop
  skip signal

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

backend/windmill-worker/src/worker_flow.rs

@@ -1099,21 +1099,22 @@ pub async fn update_flow_status_after_job_completion_internal(
                     || (flow_jobs.is_some() && (skip_loop_failures || skip_seq_branch_failure)))
                     && !(stop_early && stop_early_err_msg.is_some() && !skip_if_stop_early)
                 {
-                    let is_skipped = if current_module.as_ref().is_some_and(|m| m.skip_if.is_some())
-                    {
-                        sqlx::query_scalar!(
-                            "SELECT kind = 'identity' FROM v2_job WHERE id = $1",
-                            job_id_for_status
-                        )
-                        .fetch_one(db)
-                        .await
-                        .map_err(|e| {
-                            Error::internal_err(format!("error during skip check: {e:#}"))
-                        })?
-                        .unwrap_or(false)
-                    } else {
-                        stop_early && skip_if_stop_early // Mark as skipped when stop_after_if with skip_if_stopped=true
-                    };
+                    let is_skipped = (stop_early && skip_if_stop_early)
+                        || if current_module.as_ref().is_some_and(|m| m.skip_if.is_some()) {
+                            sqlx::query_scalar!(
+                                "SELECT kind = 'identity' FROM v2_job WHERE id = $1",
+                                job_id_for_status
+                            )
+                            .fetch_optional(db)
+                            .await
+                            .map_err(|e| {
+                                Error::internal_err(format!("error during skip check: {e:#}"))
+                            })?
+                            .flatten()
+                            .unwrap_or(false)
+                        } else {
+                            false
+                        };
                     success = true;
                     (
                         true,