Ensure all searchable terms are non-null non-empty.

osu.Game/Database/BeatmapMetadata.cs

@@ -1,6 +1,7 @@
 // Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
 // Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
 
+using System.Linq;
 using SQLite.Net.Attributes;
 
 namespace osu.Game.Database
@@ -31,6 +32,6 @@ public class BeatmapMetadata
             TitleUnicode,
             Source,
             Tags
-        };
+        }.Where(s => !string.IsNullOrEmpty(s)).ToArray();
     }
 }