Allow exporter to connect to agh instance with no credentials

[regiolis]

docker run
-e 'ADGUARD_SERVERS=http://localhost:3180'
-e 'ADGUARD_USERNAMES='
-e 'ADGUARD_PASSWORDS='
-e 'INTERVAL=15s'
-p 9618:9618
ghcr.io/henrywhitaker3/adguard-exporter:latest
panic: no usernames supplied

goroutine 1 [running]:
main.main()
/home/runner/work/adguard-exporter/adguard-exporter/main.go:20 +0x340

Allow exporter to connect to agh instance with no credentials