[libcxx] Fix default argument for merge_archives.py -L flag

[arichardson]

Summary

If we use the default of None, we get a python exception in
find_and_diagnose_missing() instead of printing a sensible error message.

libcxx/utils/merge_archives.py

parser.add_argument(
         '-v', '--verbose', dest='verbose', action='store_true', default=False)
     parser.add_argument(
         '-o', '--output', dest='output', required=True,
         help='The output file. stdout is used if not given',
         type=str, action='store')
     parser.add_argument(
         '-L', dest='search_paths',
         help='Paths to search for the libraries along', action='append',
-        nargs=1)
+        nargs=1, default=[])
     parser.add_argument(
         '--ar', dest='ar_exe', required=False,
         help='The ar executable to use, finds \'ar\' in the path if not given',
         type=str, action='store')
     parser.add_argument(
         '--use-libtool', dest='use_libtool', action='store_true', default=False)
     parser.add_argument(
         '--libtool', dest='libtool_exe', required=False,