Sanitize null bytes in eval'ed text

[yut23]

Both vim and neovim error out if we try to evaluate a string with a null byte in it. It appears that they use <NL> to represent <Nul> in several places, so replace any null bytes in text passed to vim_helper.eval() with newlines.

In my testing, I copied text from a file containing a null byte and pasted it into the commands that were giving errors, and found that the null byte does get treated as a word boundary in that case:

ultisnips/pythonx/UltiSnips/snippet/definition/base.py

Lines 345 to 347 in 24a3ebb

	# Require a word boundary between prefix and suffix.
	boundary_chars = escape(words_prefix[-1:] + words_suffix[:1], r"\"")
	match = vim_helper.eval('"%s" =~# "\\\\v.<."' % boundary_chars) != "0"

ultisnips/pythonx/UltiSnips/snippet/definition/base.py

Lines 388 to 392 in 24a3ebb

	# Trim non-empty prefix up to word boundary, if present.
	qwords = escape(words, r"\"")
	words_suffix = vim_helper.eval(
	'substitute("%s", "\\\\v^.+<(.+)", "\\\\1", "")' % qwords
	)

Fixes #1386.

[yut23]

I'll see if I can add some tests tomorrow.

[SirVer]

Thank you for your contribution! A test would indeed be highly valued.

One question: Why newline though? Why not space or empty text? Is there a rational for the one over the other?

[yut23]

Ah, it looks like markdown interpreted the character names as html tags and hid them. See :h NL-used-for-Nul/:helpg <Nul> for more details. I think a space would also work fine for the commands that were giving errors, but I figured it would be better to be able to distinguish between actual spaces and null bytes in general. I know snippet triggers can have spaces, but I don't think they can have multiple lines.

I've been pretty busy this week, but I should be able to add tests this weekend.

[yut23]

I've added some tests and checked that they fail without this change (yut23#1). I also found that any failing CI tests show up as passing in the GitHub interface, which is fixed in #1541 (I also included the commit in this PR, so it should be merged before this one).

[SirVer]

What an exemplary contribution to open source, thank you so much!