Bugfix: Handle Merge Requests without Changes (#136)

This MR makes the plugin throw the correct error when someone tries to open up the reviewer without any changes

lua/gitlab/reviewer/diffview.lua

@@ -16,6 +16,11 @@ M.open = function()
     return
   end
 
+  if diff_refs.base_sha == "" or diff_refs.head_sha == "" then
+    u.notify("Merge request contains no changes", vim.log.levels.ERROR)
+    return
+  end
+
   vim.api.nvim_command(string.format("DiffviewOpen %s..%s", diff_refs.base_sha, diff_refs.head_sha))
   M.tabnr = vim.api.nvim_get_current_tabpage()