fix(clipboard): exception when pasting into an empty directory when root_folder_label = false

[dotbillu]

Issue

• p/gp in a empty directory returns nil value as focused line is blank which was failing do_paste.

E5108: Lua: ...azy/nvim-tree.lua/lua/nvim-tree/actions/fs/clipboard.lua:360: attempt to index local 'node' (a nil value)                                                     
stack traceback:                                                                                                                                                             
        ...azy/nvim-tree.lua/lua/nvim-tree/actions/fs/clipboard.lua:360: in function 'do_paste'                                                                              
        ...azy/nvim-tree.lua/lua/nvim-tree/actions/fs/clipboard.lua:451: in function 'paste'                                                                                 
        ...share/nvim/lazy/nvim-tree.lua/lua/nvim-tree/api/impl.lua:165: in function <...share/nvim/lazy/nvim-tree.lua/lua/nvim-tree/api/impl.lua:165>    

Fix

• simply fall back to self.explorer.

[alex-courtis]

Replicate with root_folder_label = false. Assuming that is the use case.

Next time please include config in the issue/PR description for clearer replication as per bug report template or Troubleshooting

[alex-courtis]

Many thanks for your contribution! Fix works as intended.