fix show_deleted when buffer not in cache

[garcia5]

I would get an error when running gs.toggle_deleted(true) from a hydra.nvim setup function.

Error message:

Error executing vim.schedule lua callback: ...cal/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/async.lua:105: The coroutine failed with this message: ...l/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/manager.lua:274: bad argument #1 to 'ipai
rs' (table expected, got nil)                                                                                                                                                                                                                
stack traceback:                                                                                                                                                                                                                             
        [C]: in function 'ipairs'                                                                                                                                                                                                            
        ...l/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/manager.lua:274: in function 'update_show_deleted'                                                                                                                                   
        ...l/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/manager.lua:324: in function 'fn'                                                                                                                                                    
        .../share/nvim/lazy/gitsigns.nvim/lua/gitsigns/debounce.lua:76: in function 'update'                                                                                                                                                 
        ...l/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/actions.lua:1294: in function <...l/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/actions.lua:1288>                                                                                     
stack traceback:                                                                                                                                                                                                                             
        [C]: in function 'error'                                                                                                                                                                                                             
        ...cal/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/async.lua:105: in function 'cb'                                                                                                                                                    
        ...cal/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/async.lua:140: in function 'cb'                                                                                                                                                    
        ...l/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/manager.lua:48: in function <...l/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/manager.lua:46>

My hydra config

I can't quite figure out how to reliably trigger this error, can spend more time investigating if this workaround isn't desirable