Fix bad modRequest in flight entry manifest (#68888)

### What

While going through the logic of flight manifest, found the `modRequest`
sometimes accidentally become `"undefined"` string due to composing with
another potential undefined value. This caused extra traversal, fixing
this will improve a bit on the performance.

packages/next/src/build/webpack/plugins/flight-client-entry-plugin.ts

@@ -644,11 +644,11 @@ export class FlightClientEntryPlugin {
       const isCSS = isCSSMod(mod)
 
       const modPath: string = mod.resourceResolveData?.path || ''
+      const modQuery = mod.resourceResolveData?.query || ''
       // We have to always use the resolved request here to make sure the
       // server and client are using the same module path (required by RSC), as
       // the server compiler and client compiler have different resolve configs.
-      let modRequest: string | undefined =
-        modPath + mod.resourceResolveData?.query
+      let modRequest: string = modPath + modQuery
 
       // Context modules don't have a resource path, we use the identifier instead.
       if (mod.constructor.name === 'ContextModule') {