checkupdates: avoid crash with --auto and None CVC

checkupdates: avoid crash with --auto and None CVC

Reproducible via `fdroid checkupdates --auto subreddit.android.appstore`
at fdroiddata HEAD (e76449ab).

WARNING: ...subreddit.android.appstore : Couldn't find package ID
CRITICAL: Unknown exception found!
Traceback (most recent call last):
  File "/home/mvdan/.bin/fdroid", line 147, in <module>
    main()
  File "/home/mvdan/.bin/fdroid", line 124, in main
    mod.main()
  File "/home/mvdan/git/fsr/fdroidserver/checkupdates.py", line 571, in main
    checkupdates_app(app)
  File "/home/mvdan/git/fsr/fdroidserver/checkupdates.py", line 469, in checkupdates_app
    if int(build.vercode) >= int(app.CurrentVersionCode):
TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType'