Evaluation of unseen user in BPRMF

Hi, here is trivial one  [79686af](https://github.com/ummae/MyMediaLite/commit/79686af166b4f8eb7f48166e06bbfed6e1fffd9b) 

Passing null parameter to HashSet when training_user_matrix[user_id] == null
===> ERROR: Argument cannot be null.
Parameter name: collection at System.Collections.Generic.HashSet1[System.Int32]..ctor (IEnumerable1 collection, IEqualityComparer1 comparer) [0x0006d] in /data1/daum/kskim/program/mono-2.10.8/mcs/class/System.Core/System.Collections.Generic/HashSet.cs:123
at System.Collections.Generic.HashSet1[System.Int32]..ctor (IEnumerable`1 collection) [0x00000] in :0
at MyMediaLite.Eval.Items+c__AnonStoreyA.<>m__14 (Int32 user_id) [0x0002e] in /data1/daum/kskim/mymedialite.git/src/MyMediaLite/Eval/Items.cs:146