fix sql error when initializing models with empty Number property. It should return NULL when a number is empty

lib/adapters/postgres.js

@@ -164,7 +164,18 @@ PG.prototype.toDatabase = function (prop, val) {
             return this.toDatabase(prop, val[0]) + ' AND ' + this.toDatabase(prop, val[1]);
         }
     }
-    if (prop.type.name === 'Number') return val;
+    if (prop.type.name === 'Number') {
+      if (!val) {
+          if( prop.autoIncrement ) {
+              return 'DEFAULT';
+          }
+          else {
+              return 'NULL';
+          }
+      }
+      return val
+    };
+
     if (prop.type.name === 'Date') {
         if (!val) {
             if( prop.autoIncrement ) {