Decrypt file name feature: Always fails with `UnsupportedOperationException`

[infeo]

Version: 2.9.0-beta1

Description: In #263 an API was added to decrypt the filename of a given ciphertext path. This is only possible, if the dirId.c9r file is present. But event if it is present and valid, the API always throws a UnsupportedOperationException.

Excerpt of Stacktrace:

java.lang.UnsupportedOperationException: Directory does not have a dirid.c9r file.
	at org.cryptomator.cryptofs@2.8.0/org.cryptomator.cryptofs.FileNameDecryptor.decryptFilenameInternal(FileNameDecryptor.java:48)
	at org.cryptomator.cryptofs@2.8.0/org.cryptomator.cryptofs.FileNameDecryptor.decryptFilename(FileNameDecryptor.java:39)
	at org.cryptomator.cryptofs@2.8.0/org.cryptomator.cryptofs.CryptoFileSystemImpl.getCleartextName(CryptoFileSystemImpl.java:158)
	at org.cryptomator.desktop@1.16.0-SNAPSHOT/org.cryptomator.common.vaults.Vault.getCleartextName(Vault.java:422)