fix: avoid propagating empty context_lines

[Freyskeyd]

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Description

This PR fixes an issue where sometime the ctx_lines used to define the height were empty, triggering an error about invalid height:

lua/treesitter-context/render.lua:51: 'height' key must be a positive Integer

The fix is checking if the context_lines are not empty, if it is, the close method is called.

PR Checklist:

• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] I have added or updated tests that comprehensively prove my change is effective or that my feature works

[Freyskeyd]

Hey @lewis6991 !

Any chance to make this problem fix in some way? It is pretty annoying on a daily basis ahah. I'm ready to make changes or update to this PR if needed !

[lewis6991]

I've done some changes recently is this still an issue? If so do you have a way to reproduce the problem?

[Freyskeyd]

@lewis6991 I'll update and let you know if I encounter the issue again, thanks !

[lewis6991]

I'll assume this isn't an issue any more. Let me know if it is.