fix(apply_shim): add nil check for opts

[TheLeoP]

#175 does not check for possible nil value of opts on add_shim (that may come from :h dressing_get_config()). Add nil check

Context

#175 added apply_shim to maintain backwards compatibility, but it doesn't check if opts is nil before trying to access it. It may be nil since :h dressing_get_config() allows for a nil return value.

Description

A nil check is added

Test Plan

My config was broken after #175 since I have the following on my oil setup.

{
  -- ...
  input = {
    get_config = function(opts)
      if opts.prompt == "cmd: " then
        return {
          -- some custom options
        }
      end
    end,
  },
  -- ...
}

The function returns nil to fallback to the default config whe prompt is not cmd: . With this config, calling vim.ui.input({}, function() end) resulted in an error before this PR and works as expected after it.

[stevearc]

Thanks for the fix!