fix restart opts not handling nil value

[ArtAndreev]

I started to make some custom mappings and created the next mapping:

vim.keymap.set('n', '<F4>', function() require('dap').restart() end)

Then I manually tested it and got and error:

E5108: Error executing lua: ...l/share/nvim/site/pack/packer/start/nvim-dap/lua/dap.lua:699: attempt to index local 'opts' (a nil value)
stack traceback:
        ...l/share/nvim/site/pack/packer/start/nvim-dap/lua/dap.lua:699: in function 'restart'
        /Users/artyom/.config/nvim/lua/core/mappings.lua:57: in function </Users/artyom/.config/nvim/lua/core/mappings.lua:57>

All other mappings have optional opts and I don't have problems with them.

This PR enables optional opts like config for restart function.