fix: quote empty arguments and escape apostrophes when building a shell command line

[acinader]

quoteForCommandLine has two defects, both of which lose or mangle an argument on
its way to a shell command line.

Empty arguments vanish. It returns a string unchanged when it contains no
quote, whitespace or shell character. An empty string contains none, so it comes
back unquoted and unwords drops it:

unwords (map quoteForCommandLine ["a", "", "b"])  ==  "a  b"

Apostrophes break the quoting. When an argument does need quoting it is
wrapped in single quotes, and escapeSingleQuotes escaped an embedded apostrophe
as \'. A backslash is not an escape inside single quotes in POSIX sh, so the
quote closes at the apostrophe and the line no longer parses:

argument   it's

before     sent to sh as  'it\'s'    -> unterminated quote, sh rejects the line
after      sent to sh as  'it'\''s'  -> addon receives [it's]

Both are fixed in quoteForCommandLine itself: an empty string becomes '', and
an apostrophe uses the POSIX idiom of closing the quote, emitting it, and
reopening.

What this affects

Every caller that builds a shell command line rather than passing an argv list:
Run.hs for addons under run and repl, and the ! shell alias path in
Cli.hs.

$ cat cmds.txt
zzzz a "" b

before   hledger run -f /dev/null cmds.txt    argv: [-f] [/dev/null] [a] [b]
after                                         argv: [-f] [/dev/null] [a] [] [b]

An apostrophe in a payee or description is common enough to hit in ordinary use —
hledger ui "Trader Joe's" fails on main.

Windows

This does not close the Windows gap. quoteForCommandLine has no platform branch,
and cmd.exe does not recognise single quotes as a quoting character —
shellQuoteIfNeeded's docstring says exactly that, and uses double-quote escaping
there instead. So on Windows '' would arrive as two literal characters rather
than an empty argument. Closing that would mean making quoteForCommandLine
platform-aware.

I have a CI probe showing empty arguments survive on Windows when quoted as "",
but that is a different form from what this PR emits, so I am not claiming the
Windows half is fixed. I'll confirm the actual behaviour separately.

Tests

run.test 31 covers an empty argument through run, 32 through repl, and 33 an
argument containing an apostrophe. A doctest covers quoteForCommandLine directly.
All fail on main and pass with the fix.

Full functional suite 1735/1735, doctests 297/297, builds warning-free under -Werror.

Provenance

Found while testing #2699, but independent of it: the defect is in hledger-lib
and predates that PR. It stands on its own against main and can land in any order.

The apostrophe half was also reached independently in the review session linked
from #2699 (finding 5), which suggested fixing both in quoteForCommandLine
together rather than special-casing them at the call sites — which is what this
now does.

Ready for merge but marked draft to comply with pr policy.

AI usage: Claude Opus 5, ~15k output tokens

[acinader]

find a way to test this on windows