GUACAMOLE-839: Correct WildcardURIGuacamoleProperty to correctly handle missing (null) properties.

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-ssl/src/main/java/org/apache/guacamole/auth/ssl/conf/WildcardURIGuacamoleProperty.java

@@ -45,6 +45,9 @@ public abstract class WildcardURIGuacamoleProperty extends URIGuacamoleProperty
     @Override
     public URI parseValue(String value) throws GuacamoleException {
 
+        if (value == null)
+            return null;
+
         // Verify wildcard prefix is present
         Matcher matcher = WILDCARD_URI_PATTERN.matcher(value);
         if (matcher.matches()) {