nvim: statusline: Fix git status being nil

signs.add/change/delete can be nil for a file not yet being tracked
with git. This was broken in 703d23c.

nvim/.config/nvim/after/plugin/statusline.lua

@@ -143,11 +143,11 @@ M.get_git_status = function(self)
     and string.format(
       " %s+%s %s~%s %s-%s",
       "%#DiffAdded#",
-      signs.add,
+      signs.add or 0,
       "%#DiffChanged#",
-      signs.change,
+      signs.change or 0,
       "%#DiffRemoved#",
-      signs.delete
+      signs.delete or 0
     )
     or ""
 end