lander crashes for tickets that do not add packages

The lander assumed an empty "added_binaries" list would be an empty string instead of a Null.

[log]
Started by upstream project "lander_master" build number 1
originally caused by:
 Started by remote host 10.55.60.238
Building in workspace /var/lib/jenkins/jobs/lander_image_builder/workspace
[workspace] $ /bin/bash /tmp/hudson1910876454203969840.sh
+ '[' -d results ']'
+ mkdir results
+ echo '{"bsbuilder":' '{"status":' '{"building":' '"set([])",' '"failed":' '"set([])",' '"pending":' '"set([])"},' '"source_packages":' '["https://swift.canonistack.canonical.com/v1/AUTH_b262f07e0f184b6492a7fba734c345ae/ticket-1//home/chris/junk/lintian4python_0.27.1ubuntu1+0228-0933ubuntu1_source.changes",' '"https://swift.canonistack.canonical.com/v1/AUTH_b262f07e0f184b6492a7fba734c345ae/ticket-1//home/chris/junk/lintian4python_0.27.1ubuntu1+0228-0933ubuntu1.tar.gz",' '"https://swift.canonistack.canonical.com/v1/AUTH_b262f07e0f184b6492a7fba734c345ae/ticket-1//home/chris/junk/lintian4python_0.27.1ubuntu1+0228-0933ubuntu1.dsc"],' '"artifacts":' '[],' '"ppa":' '"ppa:cjohnston/ci-pool-001",' '"state":' '"COMPLETED",' '"exit":' true, '"result":' '"PASSED"},' '"master":' '{"ppa_assigner_url":' '"http://10.55.61.101:8080",' '"progress_trigger":' '"lander_master-1",' '"imgbuilder_url":' '"http://10.55.61.100:8080",' '"tr_url":' '"http://10.55.61.106:8080",' '"bsb_url":' '"http://10.55.61.96:8080",' '"request_id":' '"1",' '"ts_url":' '"http://10.55.61.132:8080",' '"request_parameters":' '{"status":' '"Not' 'started",' '"added_binaries":' null, '"updated":' '"Fri,' 28 Feb 2014 14:39:28 '+0000",' '"current_workflow_step":' '"Queued",' '"description":' '"desc",' '"created":' '"Fri,' 28 Feb 2014 14:39:15 '+0000",' '"series":' '"saucy",' '"subticket":' '[{"status":' '"New",' '"current_workflow_step":' '"New",' '"artifact":' '[{"id":' 3, '"type":' '"SPU",' '"name":' '"lintian4python_0.27.1ubuntu1+0228-0933ubuntu1_source.changes",' '"reference":' '"https://swift.canonistack.canonical.com/v1/AUTH_b262f07e0f184b6492a7fba734c345ae/ticket-1//home/chris/junk/lintian4python_0.27.1ubuntu1+0228-0933ubuntu1_source.changes",' '"resource_uri":' '"/api/v1/fullsubticketartifact/3/"},' '{"id":' 2, '"type":' '"SPU",' '"name":' '"lintian4python_0.27.1ubuntu1+0228-0933ubuntu1.tar.gz",' '"reference":' '"https://swift.canonistack.canonical.com/v1/AUTH_b262f07e0f184b6492a7fba734c345ae/ticket-1//home/chris/junk/lintian4python_0.27.1ubuntu1+0228-0933ubuntu1.tar.gz",' '"resource_uri":' '"/api/v1/fullsubticketartifact/2/"},' '{"id":' 1, '"type":' '"SPU",' '"name":' '"lintian4python_0.27.1ubuntu1+0228-0933ubuntu1.dsc",' '"reference":' '"https://swift.canonistack.canonical.com/v1/AUTH_b262f07e0f184b6492a7fba734c345ae/ticket-1//home/chris/junk/lintian4python_0.27.1ubuntu1+0228-0933ubuntu1.dsc",' '"resource_uri":' '"/api/v1/fullsubticketartifact/1/"}],' '"assignee":' '"<email address hidden>",' '"source_package_upload":' '{"version":' '"0.27.1ubuntu1+0228-0933ubuntu1",' '"sourcepackage":' '{"id":' 1, '"name":' '"lintian4python",' '"resource_uri":' '"/api/v1/sourcepackage/1/"},' '"id":' 1, '"resource_uri":' '"/api/v1/spu/1/"},' '"id":' 1, '"resource_uri":' '"/api/v1/fullsubticket/1/"}],' '"bug_id":' null, '"master_ppa":' '"ppa:ci-engineering-airline/ci-archive",' '"owner":' '"<email address hidden>",' '"title":' '"l4p' '0228-0933",' '"base_image":' '"http://cloud-images.ubuntu.com/releases/13.10/release-20131015/ubuntu-13.10-server-cloudimg-amd64-disk1.img",' '"removed_binaries":' null, '"id":' 1, '"resource_uri":' '"/api/v1/next/1/"}},' '"ppa_assigner":' '{"ppa":' '"ppa:cjohnston/ci-pool-001",' '"result":' '"PASSED",' '"location":' '"http://10.55.61.101:8080/api/v1/ppa/1/"}}'
+ cat
++ sed 's/"//g'
++ grep -Po '(?<=ts_url": )"http:.*?"'
++ cat params.json
+ /srv/lander_jenkins_sub/lander/bin/lander_service_wrapper.py --input-file params.json --output-file results/params.json --service image_builder
14:48:44 DEBUG lander_service_wrapper: Executing service wrapper: image_builder
14:48:44 ERROR root: Unexpected error: object of type 'NoneType' has no len()
Traceback (most recent call last):
  File "/srv/lander_jenkins_sub/lander/bin/lander_service_wrapper.py", line 327, in main
    results = handler(args)
  File "/srv/lander_jenkins_sub/lander/bin/lander_service_wrapper.py", line 226, in _handle_image_builder
    'package_list': _get_binary_packages(ticket, request_parameters),
  File "/srv/lander_jenkins_sub/lander/bin/lander_service_wrapper.py", line 200, in _get_binary_packages
    if len(added_list) > 0:
TypeError: object of type 'NoneType' has no len()
Build step 'Execute shell' marked build as failure
Archiving artifacts
Triggering a new build of lander_archiver #2
Finished: FAILURE