fix(document): handle validating deeply nested subdocuments underneath nested paths with required: false

Fix #12021

Author: vkarpov15

lib/document.js

@@ -1409,7 +1409,11 @@ Document.prototype.$set = function $set(path, val, type, options) {
       // later in `$__set()` because we don't take `_doc` when we iterate through
       // a single nested doc. That's to make sure we get the correct context.
       // Otherwise we would double-call the setter, see gh-7196.
-      val = schema.applySetters(val, this, false, priorVal, options);
+      if (options != null && options.overwriteImmutable) {
+        val = schema.applySetters(val, this, false, priorVal, { overwriteImmutable: true });
+      } else {
+        val = schema.applySetters(val, this, false, priorVal);
+      }
     }
 
     if (Array.isArray(val) &&

test/document.test.js

@@ -11522,7 +11522,7 @@ describe('document', function() {
       {
         nested: {
           type: SubSubSchema,
-          required: false
+          required: false // <-- important
         }
       },
       { _id: false }