Client side idStringify error (objects other than ObjectID) also with _id="" after core packages upgrade

[jankapunkt]

The version of Meteor showing the problem.
Meteor version: 1.5.1

Packages updated:

accounts-base          upgraded from 1.3.1 to 1.3.2
babel-compiler         upgraded from 6.19.4 to 6.20.0
boilerplate-generator  upgraded from 1.1.1 to 1.2.0
ddp-client             upgraded from 2.0.0 to 2.1.0
ejson                  upgraded from 1.0.13 to 1.0.14
minifier-js            upgraded from 2.1.1 to 2.1.2
minimongo              upgraded from 1.2.1 to 1.3.0
modules                upgraded from 0.9.4 to 0.10.0
mongo                  upgraded from 1.1.22 to 1.2.0
mongo-dev-server       added, version 1.0.1
promise                upgraded from 0.8.9 to 0.9.0
webapp                 upgraded from 1.3.17 to 1.3.18

The last version of Meteor where the problem did not occur (if applicable)
Same version before the update.

The operating system you're running Meteor on.
MacOs, but problem also occurs on Ubuntu 16.04.2 LTS Linux local and production.

The expected behavior
Before the update there were no problems with calling collection.findOne("") or collection.findOne({_id:""}). I expect the function findOne to return a falsey value, such as null or undefined.

The actual behavior.
Throws error on collection.findOne({_id:""})

Error: Meteor does not currently support objects other than ObjectID as ids
idStringify — 436bf8d602463537fc560f5794e92e560aa45dc0.js:56258
get — 436bf8d602463537fc560f5794e92e560aa45dc0.js:56013
e — 436bf8d602463537fc560f5794e92e560aa45dc0.js:57512
e — 436bf8d602463537fc560f5794e92e560aa45dc0.js:57443:156
e — 436bf8d602463537fc560f5794e92e560aa45dc0.js:57495
e — 436bf8d602463537fc560f5794e92e560aa45dc0.js:57393
e — 436bf8d602463537fc560f5794e92e560aa45dc0.js:45:14769
e — 436bf8d602463537fc560f5794e92e560aa45dc0.js:57658:119
findOne — 436bf8d602463537fc560f5794e92e560aa45dc0.js:62014

Is this behavior to be expected now with the updated packages?

I checked the mongo 2.6 merge and it seems, that it introduces new _id handling?

A simple reproduction!
Create a new Meteor project with the above package versions and call findOne({_id:""}) on a arbitrary client side collection.

[klaussner]

Here is a reproduction repo: https://github.com/klaussner/meteor-issues/tree/9060. It looks like this bug was introduced in #8893, specifically by this commit. The following line assigns the selector object instead of the _id to this._selectorId if the _id in the selector object is an empty string:

meteor/packages/minimongo/cursor.js

Line 14 in 92555c3

this._selectorId = selector._id || selector;

Unminified stack trace:

Uncaught Error: Meteor does not currently support objects other than ObjectID as ids
    at _IdMap.MongoID.idStringify [as _idStringify] (mongo-id.js?hash=345d169d517353f8146292b4abd24061721f8b26:104)
    at _IdMap.get (id-map.js?hash=c7aea8dfa2bf46ff2ae0aa6c6cf09e36abc61d07:46)
    at Cursor._getRawObjects (cursor.js:377)
    at Cursor.forEach (cursor.js:103)
    at Cursor.fetch (cursor.js:68)
    at LocalCollection.findOne (local_collection.js:86)
    at Mongo.Collection.findOne (collection.js:334)
    at index.js (index.js:4)
    at fileEvaluate (modules-runtime.js?hash=8587d188e038b75ecd27ed2469a52b269e38fb62:343)
    at require (modules-runtime.js?hash=8587d188e038b75ecd27ed2469a52b269e38fb62:238)