Failure to copy `OpenVRSettings.asset` when build path extension is empty

[kitlith]

In some cases, you can induce unity to build a game without providing a file extension. For instance, when using https://game.ci and building for linux, it currently produces an executable without a file extension by default.

This breaks here:

unity-xr-plugin/com.valve.openvr/Editor/OpenVRSettingsCopier.cs

Line 73 in 3aa1726

string buildName = buildPath.Name.Replace(buildPath.Extension, "");

because String.Replace will throw an exception if the first argument is empty.

Ideally, that would be guarded against by not doing the replace if buildPath.Extension is empty.