NULL values in columns with a numeric type (all of int8, int16 etc, single, double) are returned as 0.
This is due to a bug/quirk of the Osisoft PI JDBC driver, which is hard-coded to return false from the wasNull method.

On the Teiid side, the code in JDBCExecutionFactory.retrieveValue() assumes (completely reasonably) that the wasNull method is implemented correctly:

case DataTypeManager.DefaultTypeCodes.INTEGER:  {
    int value = results.getInt(columnIndex);                    
    if(results.wasNull()) {
        return null;
    }
    return Integer.valueOf(value);
}

I managed to workaround the bug in the PI JDBC driver by overriding the retrieveValue() in PIExecutionFactory and replacing the calls to wasNull like this:

case DataTypeManager.DefaultTypeCodes.INTEGER: {
    int value = results.getInt(columnIndex);
    if (results.getObject(columnIndex) == null) {
        return null;
    }
    return Integer.valueOf(value);
}

but that probably isn't the best solution.