empty user-agent and logs

[Christos Chatzaras]

curl -A "-" https://hostname/index.php <https://hostname/index.php>

and

curl -A "" https://hostname/index.php <https://hostname/index.php>

are logged with:

xxx.xxx.xxx.xxx - - [14/Sep/2017:22:47:09 +0300] "GET /index.php HTTP/1.1" 200 26039 "-" "-"

There is not difference if there is an empty user-agent or a user-agent with a dash.

Any idea it shows the empty user agents with dash?
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