Fixed ShorteningFileSystem/Folder

* ShorteningFileSystem now exists after instantiation
* Creation of ShorteningFolder does no longer fail

main/filesystem-nameshortening/src/main/java/org/cryptomator/filesystem/shortening/ShorteningFileSystem.java

@@ -3,10 +3,11 @@ package org.cryptomator.filesystem.shortening;
 import org.cryptomator.filesystem.FileSystem;
 import org.cryptomator.filesystem.Folder;
 
-class ShorteningFileSystem extends ShorteningFolder implements FileSystem {
+public class ShorteningFileSystem extends ShorteningFolder implements FileSystem {
 
 	public ShorteningFileSystem(Folder root, Folder metadataRoot, int threshold) {
 		super(null, root, "", new FilenameShortener(metadataRoot, threshold));
+		create();
 	}
 
 }

main/filesystem-nameshortening/src/main/java/org/cryptomator/filesystem/shortening/ShorteningFolder.java

@@ -55,7 +55,7 @@ class ShorteningFolder extends DelegatingFolder<ShorteningFolder, ShorteningFile
 		if (exists()) {
 			return;
 		}
-		parent().get().create();
+		parent().ifPresent(Folder::create);
 		if (shortener.isShortened(shortenedName())) {
 			shortener.saveMapping(name(), shortenedName());
 		}