Reinclude accidentally removed check if implicit token is null

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/AuthenticationProviderService.java

@@ -152,12 +152,14 @@ public class AuthenticationProviderService implements SSOAuthenticationProviderS
 
         if (confService.getFlowType().equals("implicit")) {
             String token = credentials.getParameter(IMPLICIT_TOKEN_PARAMETER_NAME);
+            if (token != null) {
                 JwtClaims claims = tokenService.validateToken(token);
                 if (claims != null) {
                     username = tokenService.processUsername(claims);
                     groups = tokenService.processGroups(claims);
                     tokens = tokenService.processAttributes(claims);
                 }
+            }
         } else {
             String verifier = null;
             if (confService.isPKCERequired()) {