fixed enter key throwing error

[TobinPalmer]

When you have no buttons and press the enter key, an error gets thrown. This simple check fixes it.

Example Config:

return {
  'goolord/alpha-nvim',
  config = function()
    local alpha = require 'alpha'
    local dashboard = require 'alpha.themes.dashboard'
    local api = vim.api

    dashboard.section.buttons.val = {}

    dashboard.section.header.opts.hl = 'Include'
    dashboard.section.buttons.opts.hl = 'Macro'
    dashboard.section.footer.opts.hl = 'Type'
    dashboard.opts.opts.noautocmd = true

    alpha.setup(dashboard.opts)
  end,
}

Error Thrown: E5108: Error executing lua: ...rs/tobin/.local/share/nvim/lazy/alpha-nvim/lua/alpha.lua:40: attempt to call a nil value
stack traceback:
...rs/tobin/.local/share/nvim/lazy/alpha-nvim/lua/alpha.lua:40: in function 'press'
...rs/tobin/.local/share/nvim/lazy/alpha-nvim/lua/alpha.lua:705: in function <...rs/tobin/.local/share/nvim/lazy/alpha-nvim/lua/alpha.lua:70 5>

[goolord]

honestly this just feels like a waste of cpu cycles, if you really want an alpha theme with no buttons just make an empty one bound to nothing somewhere