This msy be useful when ingesting logs from different sources, which store the log message in different fields.
For example, `_msg_field=message,event.data,some_field` will get log message from the first non-empty field:
`message`, `event.data` and `some_field`.
(cherry picked from commit ed73f8350b)
Empty fields are treated as non-existing fields by VictoriaLogs data model.
So there is no sense in returning empty fields in query results, since they may mislead and confuse users.
(cherry picked from commit bac193e50b)
