Fix environment getting

Author: niezbop

lib/u3d/unity_versions.rb

@@ -137,7 +137,8 @@ class << self
       def list_available(os: nil)
         os ||= U3dCore::Helper.operating_system
 
-        additional_pages = ENV['U3D_ADDITIONAL_PAGES'].split(',') || []
+        additional_pages_env = ENV['U3D_ADDITIONAL_PAGES']
+        additional_pages = additional_pages_env ? additional_pages_env.split(',') : []
 
         case os
         when :linux