jsonencoder: support empty dynamic
jsonencoder: support empty dynamic

Change-Id: If9cba8adc8e7063d94469ceeb824e075d70003cd
Reviewed-on: http://gerrit.aldebaran.lan:8080/15265
Reviewed-by: cgestes <ddb78fe00abb42f91254c16a0860e38eb0781789@aldebaran-robotics.com>
Tested-by: cgestes <ddb78fe00abb42f91254c16a0860e38eb0781789@aldebaran-robotics.com>

bsautron/libqi,aldebaran/libqi,aldebaran/libqi,vbarbaresi/libqi,aldebaran/libqi