Support blank OAuth client secret

src/main/java/org/genesys2/client/oauth/GenesysClient.java

@@ -92,7 +92,11 @@ public class GenesysClient {
 	 */
 	public GenesysClient(final String baseUrl, final String clientId, final String clientSecret, final String callbackUrl, String scope) {
 		this.baseUrl = baseUrl;
-		service = new ServiceBuilder(clientId).apiSecret(clientSecret).callback(callbackUrl).scope(scope).build(new GenesysApi(baseUrl));
+		ServiceBuilder serviceBuilder = new ServiceBuilder(clientId).callback(callbackUrl).scope(scope);
+		if (StringUtils.isNotBlank(clientSecret)) {
+			serviceBuilder.apiSecret(clientSecret);
+		}
+		service = serviceBuilder.build(new GenesysApi(baseUrl));
 	}
 
 	/**