Fix bug I introduced in last commit with ${empty:-} (no quotes).

Fixes issue #41.

osh/word_parse.py

@@ -149,8 +149,9 @@ def _ReadVarOpArg(self, arg_lex_mode, eof_type=Id.Undefined_Tok,
    # potential problem of not having spids.
    #
    # NOTE: empty_ok is False only for the PatSub pattern, which means we'll
    # return a CompoundWord(parts=[]), which fails later.
    if not w.parts and empty_ok:
    # return a CompoundWord with no parts, which is explicitly checked with a
    # custom error message.
    if not w.parts and arg_lex_mode == LexMode.VS_ARG_DQ and empty_ok:
      w.parts.append(ast.SingleQuotedPart())
    return w

spec/var-sub-quote.test.sh

@@ -14,11 +14,16 @@
# LexState.OUTER.  If we have "${}", then it's parsed as LexState.DQ.  That
# makes sense I guess.  Vim's syntax highlighting is throwing me off.

### :- with empty alternative
### "${empty:-}"
empty=
argv.py "${empty:-}"
# stdout: ['']

### ${empty:-}
empty=
argv.py ${empty:-}
# stdout: []

### :-
empty=''
argv.py ${empty:-a} ${Unset:-b}