Null check in search in Kibana

In the Kibana Discover section, while creating a search, can I filter with a condition like transactionId<>null. I am logging two different types of information; a) API usage, 2) online transaction information. The API usage log entries will not have the transaction id. So, can you please tell me the syntax to filter out the entries where transaction id is not null in Kibana searches?

Since it seems like these are different kinds of documents, perhaps you should have different types for them? That would make it obvious how to filter them.

Anyway, the null-check syntax is described here: