px_process_events: fix no base path being a list

This make `make extract_events` fail in CI that only runs on the
main branch and hence I wasn't aware of.

Tools/px_process_events.py

@@ -56,7 +56,7 @@ def main():
                         nargs='*',
                         help="one or more paths/files to source files to scan for events")
     parser.add_argument("-b", "--base-path",
-                        default=[""],
+                        default="",
                         metavar="PATH",
                         help="path prefix for everything passed with --src-path")
     parser.add_argument("-j", "--json",