fix: null pointer exception when sync has not happened yet

[DominicGBauer]

Description

A null pointer exception is thrown when sync has not yet happened in updateHasSynced. When initially testing I was logged in and everything worked as expected but after deleting the app I ran into this issue.

Work Done

Added a try catch to catch the case when the cursor receives a null value in:

val timestamp = internalDb.getOptional("SELECT powersync_last_synced_at() as synced_at", null) { cursor ->
    cursor.getString(0)!!
}