Add default kwarg to MetaDatabase.get()

[sloretz]

I think this fixes #720

It should fix broken rosdep CLI when a kinetic workspace is sourced. I suspect the environment is:

• There is an existing kinetic install, and it's sourced in the current terminal
• rosdep update has been successfully run in the past.
• ros_environment on kinetic does not set ROS_PYTHON_VERSION, but it does set ROS_DISTRO

Then I think what happened is:

1. Rosdep saw ROS_DISTRO, but no ROS_PYTHON_VERSION, so it tried to ask the cache which python version to use.
2. Metadatabase.get() had no data about the python version since rosdep update hasn't been run since the python_version info was added to the rosdistro index
3. Code here tried to treat the return value of None as an iterable type 💥

@acarrillo Mind trying this pull request on your system?

[acarrillo]

@sloretz yup looks good thanks for the quick fix! 👍 Performed an editable rosdep install from source and was able to rosdep update

acarrillo $ rosdep update
reading in sources list data from /etc/ros/rosdep/sources.list.d
Hit file:///etc/ros/rosdep/farmwise.yaml
Hit https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/osx-homebrew.yaml
Hit https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/base.yaml
Hit https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/python.yaml
Hit https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/ruby.yaml
Hit https://raw.githubusercontent.com/ros/rosdistro/master/releases/fuerte.yaml
Query rosdistro index https://raw.githubusercontent.com/ros/rosdistro/master/index-v4.yaml
Skip end-of-life distro "ardent"
Skip end-of-life distro "bouncy"
Add distro "crystal"
Add distro "dashing"
Add distro "eloquent"
Skip end-of-life distro "groovy"
Skip end-of-life distro "hydro"
Skip end-of-life distro "indigo"
Skip end-of-life distro "jade"
Add distro "kinetic"
Skip end-of-life distro "lunar"
Add distro "melodic"
Add distro "noetic"
updated cache in /home/acarrillo/.ros/rosdep/sources.cache

[codecov-io]

Codecov Report

Merging #721 into master will increase coverage by <.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #721      +/-   ##
==========================================
+ Coverage   76.03%   76.04%   +<.01%     
==========================================
  Files          40       40              
  Lines        3126     3127       +1     
==========================================
+ Hits         2377     2378       +1     
  Misses        749      749

Impacted Files	Coverage Δ
src/rosdep2/main.py	49.36% <0%> (ø)	⬆️
src/rosdep2/meta.py	100% <100%> (ø)	⬆️

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