<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _normalize_url(graph: BELGraph, keyword: str) -> Optional[str]: # FIXME move to utilities and unit test """Normalize a URL for the BEL graph."""

    if keyword == BEL_DEFAULT_NAMESPACE and BEL_DEFAULT_NAMESPACE not in graph.namespace_url:
        return BEL_DEFAULT_NAMESPACE_URL

    return graph.namespace_url.get(keyword)