Schema: Fix get_dynamic_subproperties detecting false namespaces
Schema: Fix get_dynamic_subproperties detecting false namespaces

Use find() instead of at() and be specific about switching to namespace
only when field_name is not found, not when there's no prefix.

Kronuz/Xapiand,Kronuz/Xapiand,Kronuz/Xapiand,Kronuz/Xapiand,Kronuz/Xapiand,Kronuz/Xapiand