Fix Java validation of SQL virtual tables with null column values

[mikir]

The validation of SQL virtual tables with null column values fails in Java with the following error:

Message: Column TestTable.anotherId type check failed (UNKNOWN doesn't match to INTEGER)!
Type: INVALID_COLUMN_TYPE

Used schema:

subtype int16 AnotherId;

sql_table TestTable using fts4
{
    sql_virtual int64 docId;
    string text;
    AnotherId anotherId;

    sql "notindexed='anotherId'";
};

To reproduce this bug, it is enough to set column anotherId to null and to use new JDBC driver.