validate dns config parameters

src/dns/gandi.js

@@ -112,6 +112,8 @@ function verifyDnsConfig(dnsConfig, fqdn, zoneName, ip, callback) {
     assert.strictEqual(typeof ip, 'string');
     assert.strictEqual(typeof callback, 'function');
 
+    if (!dnsConfig.token || typeof dnsConfig.token !== 'string') return callback(new DomainsError(DomainsError.BAD_FIELD, 'token must be a non-empty string'));
+
     var credentials = {
         token: dnsConfig.token
     };