Fix issue #4353. After destroy and reload, a 1-1 association is set to null.

lib/instance.js

@@ -312,7 +312,7 @@ Instance.prototype.set = function(key, value, options) { // testhint options:non
     } else {
       // Check if we have included models, and if this key matches the include model names/aliases
 
-      if (this.options && this.options.include && this.options.includeNames.indexOf(key) !== -1 && value) {
+      if (this.options && this.options.include && this.options.includeNames.indexOf(key) !== -1) {
         // Pass it on to the include handler
         this._setInclude(key, value, options);
         return;
@@ -454,7 +454,7 @@ Instance.prototype._setInclude = function(key, value, options) {
         value = value[0];
       }
 
-      isEmpty = value && value[primaryKeyAttribute] === null;
+      isEmpty = (value && value[primaryKeyAttribute] === null) || (value === null);
       self[accessor] = self.dataValues[accessor] = isEmpty ? null : include.model.build(value, childOptions);
     } else {
       isEmpty = value[0] && value[0][primaryKeyAttribute] === null;