Cleanup: Skip creating empty default server

[glensc]

No need to convert servers config on the initial run, i.e. when PLEX_SERVER from .env is not initialized.

/app # plextraktsync
WARNING  plextraktsync without command is deprecated. Executing "plextraktsync sync"                                                     
INFO     PlexTraktSync [unknown]                                                                                                         
WARNING  Added default server to /app/config/servers.yml                                                                                 
Please enter your Plex username or e-mail: ^C
Aborted!

/app # cat /app/config/servers.yml
servers:
  default:
    token: null
    urls:
    - null
    - null
    id: null
    config: null
/app #