Fix crash on backslash-only lines in code Black leaves as is - #5681

Description

a line that's only a backslash, right after another backslash continuation, makes Black crash on code it's supposed to leave alone:

# fmt: off
x = [1,
     2] + \
    \
    [3]
# fmt: on

that gives INTERNAL ERROR: Black produced invalid code. same thing on a # fmt: skip line and on lines outside --line-ranges. in an f-string field with =, or any t-string field, the field's source is part of the string, so it's "not equivalent to the source" instead, and with --fast Black just writes the changed string. CPython's own Lib/test/test_tstring.py hits it. it's a regression from 24.8.0, 24.4.2 handles all of these.

since #4343, transform_whitespace only turns the first backslash-newline into an NL token. the second one gets skipped, so the driver puts a plain newline in the prefix where the backslash was. regular code gets its whitespace redone so it doesn't matter there, but fmt: off/skip code and f-string fields are written straight from the prefix. now a backslash line that follows a backslash NL becomes an NL token too, so it stays in the prefix. list_comments still counts it as an empty line, so comments land where they did before.

output only changes where Black used to crash or drop the backslash, so already formatted files come out the same.

a lone backslash right after a regular newline still turns into a blank line, same as before. handling that too would also fix f"""{value / \ / =}""", but it changes blank lines after comments, so I left it out.

Checklist - did you ...

• Implement any code style changes under the --preview style, following the stability policy?
• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?