fix: quote an empty argument for the command line

hledger-lib/Hledger/Utils/String.hs

@@ -183,6 +183,7 @@ shellQuoteIfNeeded
 --
 quoteForCommandLine :: String -> String
 quoteForCommandLine s
+  | null s = "''"  -- an empty argument must be quoted, or it vanishes from the command line
   | any (`elem` s) (quotechars++whitespacechars++shellchars) = singleQuote $ escapeSingleQuotes s
   | otherwise = s
 

hledger/test/run.test

@@ -373,3 +373,13 @@ after
 $ mkdir -p $$d; printf '#!/bin/sh\nexit 3\n' >$$d/hledger-zzzz; chmod +x $$d/hledger-zzzz; PATH=$$d:$PATH hledger run -f /dev/null -- echo one -- zzzz -- echo two; s=$?; rm -rf $$d; exit $s
 one
 >= 3
+
+# ** 31. run passes an empty argument to an addon, as the command line does.
+$ mkdir -p $$d; printf '#!/bin/sh\nprintf "argv:"; for a in "$@"; do printf " [%%s]" "$a"; done; echo\n' >$$d/hledger-zzzz; chmod +x $$d/hledger-zzzz; printf 'zzzz a "" b\n' >$$f; PATH=$$d:$PATH hledger run -f /dev/null $$f; rm -rf $$d $$f
+argv: [-f] [/dev/null] [a] [] [b]
+
+# ** 32. repl passes an empty argument to an addon, as the command line does.
+<
+zzzz a "" b
+$ sh -c 'D=$(mktemp -d "${TMPDIR:-/tmp}/hledger-addon.XXXXXX"); J=$(mktemp "${TMPDIR:-/tmp}/hledger-addonj.XXXXXX"); printf "#!/bin/sh\nprintf argv:; for a in \"\$@\"; do printf \" [%%s]\" \"\$a\"; done; echo\n" > "$D/hledger-zzzz"; chmod +x "$D/hledger-zzzz"; printf "2017-01-01 x\n  a  1\n  b\n" > "$J"; PATH="$D:$PATH" hledger repl --no-conf -f "$J"; rm -rf "$D" "$J"'
+> /argv: \[-f\] \[.*\] \[a\] \[\] \[b\]/