Prevent "Sequence contains no elements" in browser tests

[ilonatommy]

Follow up for #1221.
Contributes to dotnet/runtime#107466.
In dotnet/runtime#107259 we discovered very frequent timeouts on Windows with newer chrome version, with logs:

          [wasm test-browser] info: Closing 2 browser tabs before setting the main tab to config page and quitting.
          [wasm test-browser] fail: Error while closing browser: System.InvalidOperationException: Sequence contains no elements
          [wasm test-browser]          at System.Linq.ThrowHelper.ThrowNoElementsException()
          [wasm test-browser]          at Microsoft.DotNet.XHarness.CLI.Commands.Wasm.WasmTestBrowserCommand.InvokeInternal(ILogger logger)
          [wasm test-browser] XHarness exit code: 0

It suggests that in the try-catch block we're using LINQ function on an empty collection. It might happen if closing of tabs did not happen instantly but with a slight delay. Then the while condition was still true but when we call Last(), windowHandles is already an empty collection.
This should fix it.

[ilonatommy]

/ba-g Android failures are not connected