fix: use "" as flag to remove default mappings (#1011)

doc/nvim-tree-lua.txt

@@ -641,7 +641,7 @@ Defaults to:
 The `list` option in `view.mappings.list` is a table of
 
 - key can be either a string or a table of string (lhs)
-- action is the name of the action, set to nil to unmap default actions
+- action is the name of the action, set to `""` to remove default action
 - action_cb is the function that will be called, it receives the node as a parameter. Optional for default actions
 - mode is normal by default
 >
@@ -656,7 +656,7 @@ The `list` option in `view.mappings.list` is a table of
       { key = {"<CR>", "o" }, action = "edit", mode = "n"},
       { key = "p", action = "print_path", action_cb = print_node_path },
       { key = "s", cb = tree_cb("vsplit") }, --tree_cb and the cb property are deprecated
-      { key = "<2-RightMouse>", action = nil },
+      { key = "<2-RightMouse>", action = "" }, -- will remove default cd action
     }