fix crash on missing project file

Author: Nsidorenco

lua/neotest-vstest/dotnet_utils.lua

@@ -150,9 +150,11 @@ function M.get_proj_info(path)
 
   if project_semaphore[proj_file] then
     semaphore = project_semaphore[proj_file]
-  else
+  else if proj_file then
     semaphore = nio.control.semaphore(1)
     project_semaphore[proj_file] = semaphore
+  else
+    return nil
   end
 
   semaphore.acquire()