chore: skip orchestrator info requests if service uri not set by orchestrator

[ad-astra-video]

What does this pull request do? Explain your changes. (required)
Update db_discovery.go to skip starting orchestrator http client if the service uri is not set by the orchestrator. At time of this commit, 71 orchestrators have not registered for the AI subnet so this skips a lot of failed http connections.

Docker build is at docker pull adastravideo/go-livepeer:ai-video-test

Specific updates (required)

• add a check to confirm a Host exists in the parsed service uri.

How did you test each of these updates (required)
Built docker file and started up gateway
startup.log

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[rickstaa]

@ad-astra-video Do we still need this now that we've merged into the master branch? Are there any orchestrators that still have their service URI set to an empty string ("") by mistake? I noticed this issue once before, but I'm not sure if any orchestrators are still affected.

[ad-astra-video]

The AI service registry is still not fully used by Orchestrators. This is just a thing to ease development starting gateways faster. For production gateways it is not likely noticed much.

I have not issue closing this but it also doesn't make sense to try to connect to a URL that is blank.

[ad-astra-video]

Counted the empty entries on last test gateway startup. There are still 59 empty urls in the AIServiceRegistry.