fix: height error while scrolling down

[Brayan-724]

Fix #431.
Scrolling down in a file when the first line is empty causes an error.

Error detected while processing WinScrolled Autocommands for "*":
Error executing lua callback: ...vim-treesitter-context/lua/treesitter-context/render.lua:51: 'height' key must be a positive Integer
stack traceback:
        [C]: in function 'nvim_open_win'
        ...vim-treesitter-context/lua/treesitter-context/render.lua:51: in function 'display_window'
        ...vim-treesitter-context/lua/treesitter-context/render.lua:354: in function 'open'
        .../lazy/nvim-treesitter-context/lua/treesitter-context.lua:50: in function 'open'
        .../lazy/nvim-treesitter-context/lua/treesitter-context.lua:111: in function 'f'
        .../lazy/nvim-treesitter-context/lua/treesitter-context.lua:27: in function <.../lazy/nvim-treesitter-context/lua/treesitter-context.lua:21