fix(blame): If it has been detached, the cache[bufnr] is nil

[JavaHello]

Error executing vim.schedule lua callback: ...cal/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/async.lua:76: The coroutine failed with this message: ...m/lazy/gitsigns.nvim/lua/gitsigns/current_line_blame.lua:154: attempt to index local 'bcache' (a nil value)
stack traceback:
	...m/lazy/gitsigns.nvim/lua/gitsigns/current_line_blame.lua: in function 'handle_blame_info'
	...m/lazy/gitsigns.nvim/lua/gitsigns/current_line_blame.lua:238: in function <...m/lazy/gitsigns.nvim/lua/gitsigns/current_line_blame.lua:184>
stack traceback:
	[C]: in function 'error'
	...cal/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/async.lua:76: in function 'cb'
	...cal/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/async.lua:113: in function 'cb'
	...cal/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/async.lua:186: in function <...cal/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/async.lua:184>