Timestamps of the first record in the day


how to get the timestamp of e.g. first record in the day?

My measurement nmap consists of records of IP addresses responding to nmap command.

I can easily get number or records group by day:
SELECT count("state_num")/4 as cnt_records, min(time) as min_time FROM nmap WHERE (ipv4_addr = '') GROUP BY time(1d)

name: nmap
time                 cnt_records min_time
----                 ----------- --------
2020-05-12T00:00:00Z 7.75
2020-05-13T00:00:00Z 5.25
2020-05-14T00:00:00Z 0
2020-05-15T00:00:00Z 0
2020-05-16T00:00:00Z 0
2020-05-17T00:00:00Z 0
2020-05-18T00:00:00Z 5.25
2020-05-19T00:00:00Z 7
2020-05-20T00:00:00Z 9.5
2020-05-21T00:00:00Z 4.25
2020-05-22T00:00:00Z 7
2020-05-23T00:00:00Z 0
2020-05-24T00:00:00Z 0.5
2020-05-25T00:00:00Z 4
2020-05-26T00:00:00Z 3.25
2020-05-27T00:00:00Z 11.5
2020-05-28T00:00:00Z 0.5

but min(time) as min_time returns null values.

How to get expected results – timestamps of the first record in the day?