Add support to logging directory setup for no verb

Explicitly support "no verb" in the logging directory setup, as opposed
to treating it as an empty string or raising an exception.

colcon_core/location.py

@@ -211,10 +211,13 @@ def create_log_path(verb_name):
     ignore_marker.touch()
 
     # create latest symlinks
-    _create_symlink(path, path.parent / f'latest_{verb_name}')
-    _create_symlink(
-        path.parent / f'latest_{verb_name}',
-        path.parent / 'latest')
+    if verb_name is None:
+        _create_symlink(path, path.parent / 'latest')
+    else:
+        _create_symlink(path, path.parent / f'latest_{verb_name}')
+        _create_symlink(
+            path.parent / f'latest_{verb_name}',
+            path.parent / 'latest')
 
 
 def _reset_log_path_creation_global():

test/test_location.py

@@ -190,6 +190,17 @@ def test_create_log_path(reset_log_path_creation_global):
         assert (log_path / subdirectory).exists()
         assert not (log_path / 'latest').is_symlink()
 
+        # check that `latest_verb` is skipped when there is no verb
+        (log_path / subdirectory).rmdir()
+        (log_path / 'latest').rmdir()
+        (log_path / 'latest_verb').unlink()
+        location._create_log_path_called = False
+        create_log_path(None)
+        assert (log_path / subdirectory).exists()
+        assert (log_path / 'latest').is_symlink()
+        assert (log_path / 'latest').resolve() == \
+            (log_path / subdirectory).resolve()
+
 
 def test__create_symlink():
     # check cases where functions raise exceptions and ensure it is being