[2.x] Fix React TS errors

[jessarcher]

This PR fixes the following TypeScript error in the React SSR build:

 resources/js/ssr.tsx(18,17): error TS2769: No overload matches this call.
  Overload 1 of 4, '(name: string & {}, params?: RouteParams<string & {}> | undefined, absolute?: boolean | undefined, config?: Config | undefined): string', gave the following error.
    Argument of type 'ParameterValue | RouteParams<string & {}> | undefined' is not assignable to parameter of type 'RouteParams<string & {}> | undefined'.
      Type 'string' is not assignable to type 'RouteParams<string & {}> | undefined'.
  Overload 2 of 4, '(name: string & {}, params?: ParameterValue | undefined, absolute?: boolean | undefined, config?: Config | undefined): string', gave the following error.
    Argument of type 'ParameterValue | RouteParams<string & {}> | undefined' is not assignable to parameter of type 'ParameterValue | undefined'.
      Type 'GenericRouteParamsObject' is not assignable to type 'ParameterValue | undefined'.
        Type 'GenericRouteParamsObject' is not assignable to type 'DefaultRoutable'.
          Property 'id' is missing in type 'GenericRouteParamsObject' but required in type '{ id: RawParameterValue; }'.
  Overload 3 of 4, '(name: undefined, params: undefined, absolute?: boolean | undefined, config?: Config | undefined): Router', gave the following error.
    Argument of type 'string & {}' is not assignable to parameter of type 'undefined'.

It also fixes a JSX/TSX syntax issue in the default welcome page that caused errors when running the SSR server.

[driesvints]

Thanks @jessarcher!