fix off by 1 error if all lines are blank

[ervandew]

when attempting to align an empty file, align_csv will try to look for a non-blank line, but in doing so it will pull from the lines array using the length + 1, leading to an error:

lua/decisive.lua:64: attempt to get length of local 'test_line' (a nil value)

This is in the context of an autocmd like so:

vim.api.nvim_create_autocmd('BufWinEnter', {
  pattern = '*.csv',
  callback = function()
    local decisive = require('decisive')
    decisive.setup({})
    decisive.align_csv({ csv_separator = ',' })
  end
})

Where the error occurs when that autocmd fires on a new csv buffer that has no content.