Null selected on taggable throws exception.

[asvae]

I'm getting an exception, that occurs when I pass down from parent selected, which is null.
Multiselect is taggable.
He're the exception:

app-cd1a3cff48.js:49506 [Vue warn]: Error when evaluating expression "
function () {
    var t = this.hideSelected ? this.options.filter(this.isNotSelected) : this.options;
    return t = this.$options.filters.filterBy(t, this.search), this.taggable && this.search.length > 0 && !this.isExistingOption(this.search) && t.unshift({ isTag: !0, label: this.search }), t;
        }
": TypeError: Cannot read property 'length' of null (found in component: <multiselect>)

Pull request fixes the issue.

[shentao]

Merged and made some small adjustments myself. Also added a test case for it. Thanks a lot @asvae! :)