Invoking file system methods with root path throws NullPointerException

[infeo]

The JDK filesystem API takes an arbitrary Path as an argument. Hence, also for the filesystem root the following is a valid method call:

Path root = fs.getPath("/");
Files.createDirectory(root);

Although the above will likely throw an exception, it should be an IOException. Currently, a NullPointerException is thrown, caused by

cryptofs/src/main/java/org/cryptomator/cryptofs/CryptoFileSystemImpl.java

Lines 630 to 635 in 59a8ec2

	void assertCleartextNameLengthAllowed(CryptoPath cleartextPath) throws FileNameTooLongException {
	String filename = cleartextPath.getFileName().toString();
	if (filename.length() > fileSystemProperties.maxCleartextNameLength()) {
	throw new FileNameTooLongException(cleartextPath.toString(), fileSystemProperties.maxCleartextNameLength());
	}
	}

.

We need some null-check for Path::getFileName. Additionally, we should check which other methods choke on the root path.