Fix update checker not working when no new primary file is found

[LostLuma]

Hey, I noticed the update checker will crash when there is no new file marked as a primary file (I'm not sure why this would happen, but it does, apparently). This PR fixes this by ignoring that specific case.

[21:18:24] [Worker-Main-1/ERROR]: Caught exception in thread Thread[#73,Worker-Main-1,5,main]
java.util.NoSuchElementException: No value present
    at java.util.Optional.get(Optional.java:143) ~[?:?]
    at com.terraformersmc.modmenu.util.ModrinthUtil.lambda$checkForUpdates$4(ModrinthUtil.java:86) ~[transformed-mod-modmenu.i0:0/:?]
    at java.util.Map.forEach(Map.java:716) ~[?:?]
    at com.terraformersmc.modmenu.util.ModrinthUtil.lambda$checkForUpdates$5(ModrinthUtil.java:79) ~[transformed-mod-modmenu.i0:0/:?]
    at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1423) ~[?:?]
    at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:387) ~[?:?]
    at java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1312) ~[?:?]
    at java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1843) ~[?:?]
    at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1808) ~[?:?]
    at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:188) ~[?:?]

[LostLuma]

Seems other people also ran into it at #607.