java: determine behavior when no API is found

[zhumin8]

Looking back - if there isn't an API, do we really want to set a transport at all? What would that even mean? (I'll approve this PR anyway as it's not introducing the issue, but we should consider just not populating it.)

Originally posted by @jskeet in #4839 (comment)

[jskeet]

(For context without consulting the PR, this was setting the transport field in .repo-metadata.json to "both" for libraries which don't have any API paths, i.e. hand-written libraires.)

[zhumin8]

The call to serviceconfig.Find and returns early if an error is encountered. So this and similar logic in java/generate.go is redundant. Will remove.