fix: treat empty IMGW warning products as no warnings

app/api/imgw/[...path]/route.ts

@@ -1,4 +1,5 @@
 import { NextResponse } from "next/server";
+import { isImgwNoProductsResponse, readImgwResponseBody } from "@/lib/imgw-empty-response";
 
 const ALLOWED_PATHS = new Set([
   "synop",
@@ -28,6 +29,12 @@ export async function GET(_: Request, context: { params: Promise<{ path: string[
       headers: { Accept: "application/json", "User-Agent": USER_AGENT },
     });
     if (!response.ok) {
+      const body = await readImgwResponseBody(response);
+      if ((resource === "warningsmeteo" || resource === "warningshydro") && response.status === 404 && isImgwNoProductsResponse(body.json)) {
+        return NextResponse.json([], {
+          headers: { "Cache-Control": "public, s-maxage=300, stale-while-revalidate=600" },
+        });
+      }
       return NextResponse.json({ error: "IMGW API jest chwilowo niedostępne." }, { status: response.status });
     }
     const data: unknown = await response.json();