fixed file read error discovered while trying to use old style authentication

guacamole-common/src/main/java/net/sourceforge/guacamole/properties/FileGuacamoleProperty.java

@@ -49,7 +49,11 @@ public abstract class FileGuacamoleProperty implements GuacamoleProperty<File> {
 
     @Override
     public File parseValue(String value) throws GuacamoleException {
-        return new File(value);
+        // having a null value is acceptable. If value is null, no error thrown.
+        if(value != null)
+            return new File(value);
+        else
+            return null;
     }
 
 }

guacamole-common/src/main/java/net/sourceforge/guacamole/properties/GuacamoleProperties.java

@@ -108,7 +108,6 @@ public class GuacamoleProperties {
      *                            guacamole.properties.
      */
     public static <Type> Type getProperty(GuacamoleProperty<Type> property) throws GuacamoleException {
-
         if (exception != null)
             throw exception;