start adding a different text for empty records for the sqlite writer.

Sadly there also seems to be another deadlock issue which I am currently
not really able to figure out - on shutdown sometimes (too often) the main
thread + all sqlite threads wait for semaphores or mutexes.

src/logging/writers/SQLite.h

@@ -46,9 +46,9 @@ private:
 	sqlite3 *db;
 	sqlite3_stmt *st;
 
-	string separator;
 	string set_separator;
 	string unset_field;
+	string empty_field;	
 	
 	AsciiInputOutput* io;
 };