Handle nil test.name (#69)

I assume the treesitter API changed (again) and caused this to start
breaking. If I try to run `debug_test()` outside of a test file, then I
get an exception for trying to concatenate a nil value. This commit just
fixes the "no test found" short-circuit clause to handle empty string or
nil.

lua/dap-go.lua

@@ -145,7 +145,7 @@ end
 function M.debug_test()
   local test = ts.closest_test()
 
-  if test.name == "" then
+  if test.name == "" or test.name == nil then
     vim.notify("no test found")
     return false
   end