"attempt to perform arithmetic on a nil value" when passing shortcut = {} for theme hyper

[Ilan Schemoul]

Describe the bug
Error executing vim.schedule lua callback: .../code/forks/dashboard-nvim/lua/d
ashboard/theme/hyper.lua:30: attempt to perform arithmetic on a nil value
stack traceback:
.../code/forks/dashboard-nvim/lua/dashboard/theme/hyper.lua:30: in fun
ction 'gen_shortcut'
.../code/forks/dashboard-nvim/lua/dashboard/theme/hyper.lua:502: in fu
nction 'callback'
.../code/forks/dashboard-nvim/lua/dashboard/theme/hyper.lua:155: in fu
nction ''
vim/_editor.lua: in function <vim/_editor.lua:0>

To Reproduce
Steps to reproduce the behavior:

    require('dashboard').setup {
      theme = 'hyper',
      config = {
        header = {},
        shortcut = {},
      }
    }

Expected behavior
No errors

[Ilan Schemoul]

I made a pull request #437