avoid NPE (even though os.name should always be defined on any JVM)

Author: overheadhunter

src/main/java/org/cryptomator/integrations/common/OperatingSystem.java

@@ -30,7 +30,7 @@ enum Value {
 		WINDOWS,
 		UNKNOWN;
 
-		private static final String OS_NAME = System.getProperty("os.name").toLowerCase();
+		private static final String OS_NAME = System.getProperty("os.name", "").toLowerCase();
 
 		public static Value current() {
 			if (OS_NAME.contains("linux")) {