asMapString
Narrows the result of asMapAny to only the entries whose value is a String, dropping every other entry — used by events that stage incomingData as HTTP headers or query parameters (SetIncomingDataToNetworkParamsHolderHeaders/...QueryParameters), where non-string values can't be forwarded as-is.
Return
the map narrowed to Map<String, String>, or null if this value isn't a map at all.