Fix bug in `rspec--include-fg-syntax-methods-p`

When called with `nil`, `cl-case` always return `nil`, even if there is an
explicit `nil` case. (see [doc](https://www.gnu.org/software/emacs/manual/html_node/cl/Conditionals.html))

For example,

```elisp
(setq myvar nil)
(cl-case myvar
  (one 1)
  (two 2)
  (nil 3)) # => nil, and not 3!
```

That is probably not what was meant by the author of
`rspec--include-fg-syntax-methods-p`. This commit changes the `nil`
option in the custom variable `rspec-snippets-fg-syntax` to the symbol`check`.

ps: Happy new year!