NameIDFormat fix (#375)

Added a conditional statement to set NameIDFormat only if identifierFormat is specified in options. This should prevent an error in AD FS when identifierFormat  set to null: #338

Author: ahavriluk

lib/passport-saml/saml.js

@@ -1249,7 +1249,10 @@ SAML.prototype.generateServiceProviderMetadata = function( decryptionCert, signi
     };
   }
 
-  metadata.EntityDescriptor.SPSSODescriptor.NameIDFormat = this.options.identifierFormat;
+  if (this.options.identifierFormat) {
+    metadata.EntityDescriptor.SPSSODescriptor.NameIDFormat = this.options.identifierFormat;
+  }
+
   metadata.EntityDescriptor.SPSSODescriptor.AssertionConsumerService = {
     '@index': '1',
     '@isDefault': 'true',