[rospack.py] Errored pkg name (#133 (comment)). 'None' causes TypeErr…

…or ('<' not supported between instances of 'NoneType' and 'License) so defining non None.

src/rospkg/rospack.py

@@ -402,8 +402,8 @@ def get_licenses(self, pkg_name, implicit=True):
         @rtype { k, [d] }
         @raise ResourceNotFound
         """
-        MSG_LICENSE_NOTFOUND_SYSPKG = "ERR"
         license_dict = defaultdict(list)
+        LICENSE_DEFINED = "license_not_found"
 
         self.get_depends(name=pkg_name, implicit=implicit)
 
@@ -414,12 +414,12 @@ def get_licenses(self, pkg_name, implicit=True):
         # Traverse for Non-ROS, system packages
         pkgnames_rosdep = self.get_rosdeps(package=pkg_name, implicit=implicit)
         for pkgname_rosdep in pkgnames_rosdep:
-            license_dict[MSG_LICENSE_NOTFOUND_SYSPKG].append(pkgname_rosdep)
+            license_dict[LICENSE_DEFINED].append(pkgname_rosdep)
 
-        # Sort pkg names in each license
+        # Sort pkg names within the set of pkgs with  each license
         for list_key in license_dict.values():
             list_key.sort()
-        # Sort license names
+        # Sort licenspe names
         licenses = OrderedDict(sorted(license_dict.items()))
         return licenses