fix: fix treesitter get_parser for nvim v0.12

[zdm]

Under nvim 0.12 vim.treesitter.get_parser() call does not throws error. It just return null.
This PR fixes this problem.

[zdm]

Close: #517

[zdm]

@numToStr
Could you please merge this PR, it is critical for neovim 0.12.

[g0ng0]

For backward compatibility maybe we should use condition not ok or not parser instead?

[zdm]

We need to check if parser is present, not OK status.
Obviously, parser will be nil if it is not available.
So current condition is enough.