pr: fix error on closing/reopening

[macovsky]

Describe what this PR does / why we need it

currently when you close/reopen pr an error pops up complaining about indexing autoMergeRequest

Does this pull request fix one issue?

NONE

Describe how to verify it

close/reopen pr to see there's no error

Checklist

• Passing tests and linting standards
• Documentation updates in README.md and doc/octo.txt

[wilkinson4]

I'm also experiencing a similar error when I'm creating a PR via :Octo pr create

|| #22768 - `Log errors when an active device is missing to see which patients it's` created successfully
|| Error executing vim.schedule lua callback: ....local/share/nvim/lazy/octo.nvim/lua/octo/ui/writers.lua:499: attempt to index field 'autoMergeRequest' (a nil value)
|| stack traceback:
|| 	....local/share/nvim/lazy/octo.nvim/lua/octo/ui/writers.lua:499: in function 'write_details'
|| 	...share/nvim/lazy/octo.nvim/lua/octo/model/octo-buffer.lua:106: in function 'render_issue'
|| 	...inson/.local/share/nvim/lazy/octo.nvim/lua/octo/init.lua:271: in function 'create_buffer'
|| 	...n/.local/share/nvim/lazy/octo.nvim/lua/octo/commands.lua:1030: in function 'cb'
|| 	...on/.local/share/nvim/lazy/octo.nvim/lua/octo/gh/init.lua:163: in function ''
|| 	vim/_editor.lua: in function <vim/_editor.lua:0>

Also, if I open a PR in a buffer the only thing displaying is the PR title, but in telescope it looks fine.

Here is my octo config:

    {
        'pwntester/octo.nvim',
        dependencies = {
            'nvim-lua/plenary.nvim',
            'nvim-telescope/telescope.nvim',
            'nvim-tree/nvim-web-devicons',
        },
        config = function()
            require "octo".setup({
                suppress_missing_scope = {
                    projects_v2 = true,
                },
                users = 'assignable'
            })
        end
    },