SftpPlugin: fix SAF implementation for gvfs/ssh

Returning NULL from readSymbolicLink() results in an exception that
closes the SFTP connection for some SSH implementations, notably the one
used by Gvfs (GSConnect).

primitive-ftpd had a similar issue, which was fixed by returning and
empty string instead of NULL.

src/org/kde/kdeconnect/Plugins/SftpPlugin/RootFile.java

@@ -88,7 +88,7 @@ class RootFile implements SshFile {
     }
 
     public String readSymbolicLink() {
-        return null;
+        return "";
     }
 
     public void createSymbolicLink(SshFile destination) {