[NekosBest] return None on failed API call, guard empty results.

[Evanroby]

_api_call returned the sent error Message instead of None on non-200 responses, so if url is None never caught it and the code went on to index into the Message, raising TypeError: 'Message' object is not subscriptable.

• _api_call now sends the error message and returns None
• embedgen/imgembedgen guard against an empty results list (e.g. API returns 200 with no data) before indexing [0]
• the empty-results check in embedgen runs before usage counters are incremented, so failed calls aren't counted as uses

[ltzmax]

Well, its mostly correct from begin of, its just the enforcement of required User-Agent that was needed since last month which i never did cause i keep forgetting
https://docs.nekos.best/getting-started/api-reference.html#user-agent

[Evanroby]

What do you want to use for the User-Agent ? You can still push commits on my branch.

[ltzmax]

What do you want to use for the User-Agent ? You can still push commits on my branch.

It was enforced by the Neko's devs to have user agents so it's needed to be done someday, I'll just use this temporary fix for now until I remember doing it