Fix tracking empty preset code in init routes

Guard against tracking an empty preset value in both /init and /init/v0
routes. When no valid preset code is in the URL, getPresetCode encodes
one from the parsed config params. The conditional spread ensures an
empty string never reaches analytics.

https://claude.ai/code/session_018Mm7WJAaSUGSqamSm3RiyC

apps/v4/app/(create)/init/v0/route.ts

@@ -25,7 +25,7 @@ export async function GET(request: NextRequest) {
     after(() => {
       track("create_open_in_v0", {
         ...result.data,
-        preset: presetCode,
+        ...(presetCode && { preset: presetCode }),
       })
     })