[wasm] Don't call .NET runtime if browser refresh script for HotReload is missing

[maraf]

The scenario that happens is:

• Application is published in debug, meaning the HotReload package is referenced
• Since it's publish, ILLink will remove all un-referenced assemblies
• Running the publish output results in calling JSExport that has been ILLinked out

To mitigate it, we don't call JSExport when browser refresh script is not used

Related to dotnet/runtime#118442

[dotnet-policy-service]

Thanks for your PR, @@maraf.
To learn about the PR process and branching schedule of this repo, please take a look at the SDK PR Guide.