Empty `Query` and null `QueryStartAt` in `.sys/query_sessions`

[artemmufazalov]

There are rows with missing fields, when displayed in UI they look odd

Request

SELECT
    UserSID, 
    QueryStartAt, 
    Query as QueryText, 
    ApplicationName
FROM `.sys/query_sessions`

In response sometimes we have rows like this

["<anonymous>", null, "", "<empty>"]

There are sessions in table, not requests, we need to filter empty value in UI: where QueryStartAt is not null

[antonkovalenko]

will be closed in #2077