It turns out the SQLite API isn't smart enough to handle nullables
directly, so we need to help it out a bit.
Stops the following from being thrown:
```
System.InvalidOperationException: Value must be set.
at Microsoft.Data.Sqlite.SqliteParameter.Bind(sqlite3_stmt stmt) = 3
at
at Microsoft.Data.Sqlite.SqliteParameterCollection.Bind(sqlite3_stmt
stmt) = 3 at
at Microsoft.Data.Sqlite.SqliteCommand.ExecuteReader(CommandBehavior
behavior)
at Microsoft.Data.Sqlite.SqliteCommand.ExecuteReader()
at
osu.Game.Beatmaps.BeatmapManager.BeatmapOnlineLookupQueue.checkLocalCache(BeatmapSetInfo
set, BeatmapInfo beatmap) in
/Users/dean/Projects/osu/osu.Game/Beatmaps/BeatmapManager_BeatmapOnlineLookupQueue.cs:line
166 = 166
```
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