feat(importer): Don't delete descriptions

If the ROS messages don't have a description for an object, keep the
current description in the Capella model unchanged. This allows adding
descriptions in the Capella model manually if they're not present in the
imported ROS messages package.

Fixes #11

capella_ros_tools/importer.py

@@ -158,9 +158,11 @@ def _convert_class(
             "find": {
                 "name": msg_def.name,
             },
-            "set": {
-                "description": msg_def.description,
-            },
+            "set": (
+                {"description": msg_def.description}
+                if msg_def.description
+                else {}
+            ),
             "sync": {
                 "properties": props,
             },