core/SQLiteConnection: Add null-check in ctor
core/SQLiteConnection: Add null-check in ctor

caseydedore/SQLite.Net-PCL,kreuzhofer/SQLite.Net-PCL,oysteinkrog/SQLite.Net-PCL,molinch/SQLite.Net-PCL,mattleibow/SQLite.Net-PCL,igrali/SQLite.Net-PCL,igrali/SQLite.Net-PCL,TiendaNube/SQLite.Net-PCL,fernandovm/SQLite.Net-PCL