mbtrading/quoteapi: Handle messages without fields

If a message contains no fields, 'MbtMessage#parse' throws a
NumberFormatException. Fix the implementation so that messages
without fields are handled properly.

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Merge pull request #151 from jvirtanen/fixes/mbt-quote-api-messages-without-fields

MBT Quote API: Handle messages without fields