fix Util::parse_to_qore_value() to support single-element lists and empty hashes

[davidnich]

ex:

david@greybeard:~/src/qore/git/qore/qlib$ qore -l Util -nX 'parse_to_qore_value("1,")'
1
david@greybeard:~/src/qore/git/qore/qlib$ qore -l Util -nX 'parse_to_qore_value("{},")'
unhandled QORE System exception thrown in TID 1 at 2018-11-24 08:04:21.921404 Sat +01:00 (CET) in <anonymous closure>() (/Users/david/src/Qorus/test/qlib/Util.qm:705, user code)
RUNTIME-OVERLOAD-ERROR: no variant matching 'parse_to_qore_value(NOTHING) can be found; the following variants were tested:
   parse_to_qore_value(string arg)
call stack:
  2: <anonymous closure>() (/Users/david/src/Qorus/test/qlib/Util.qm:705, user code)
  1: parse_to_qore_value() (<command-line>:1, user code)
david@greybeard:~/src/qore/git/qore/qlib$ qore -l Util -nX 'parse_to_qore_value("{}")'
unhandled QORE System exception thrown in TID 1 at 2018-11-24 08:04:27.147651 Sat +01:00 (CET) in <anonymous closure>() (/Users/david/src/Qorus/test/qlib/Util.qm:705, user code)
RUNTIME-OVERLOAD-ERROR: no variant matching 'parse_to_qore_value(NOTHING) can be found; the following variants were tested:
   parse_to_qore_value(string arg)
call stack:
  2: <anonymous closure>() (/Users/david/src/Qorus/test/qlib/Util.qm:705, user code)
  1: parse_to_qore_value() (<command-line>:1, user code)