For Items getter throwing exception replaced to empty collection

#### Purpose

Some methods while going through all Nodes of Tree use `Items` property getting of `ClassInformation`. It throws `NotimplementedException`.

#### Modifications

`Items` Getter returns empty collection.

#### Additional references

[MAGN-4768](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4768).

#### Reviewers

@Benglin, please take a look.