additional check of alias as string

Author: iamcharliegoddard

lib/dialect/postgres.js

@@ -689,7 +689,7 @@ Postgres.prototype.visitTable = function(tableNode) {
     txt += '.';
   }
   txt += this.quote(table.getName());
-  if(table.alias) {
+  if(typeof table.alias === 'string') {
     txt += this._aliasText + this.quote(table.alias);
   }
   return [txt];