[BUG][Java] templated servers broke simple setting of basepath

[afrische]

Bug Report Checklist

• What's the version of OpenAPI Generator used?
  4.3.0
• Have you search for related issues/PRs?
  yes

Description

when the basepath for a generated api client is changed, that change get's ignored. cause is a check for

serverIndex != null

but the variable is initialized to 0

openapi-generator version

4.3.0 , yes. regression from 4.2.3

Related issues/PRs

probably introduced from #4998

Suggest a fix

workaround is to either set the serverIndex to null explicitly, or add a single server configuration.