Distinguish ambiguous column value of `None`

[heehehe]

Resolve #479

Overview

In RowsEvent, there are three cases in which the column value is None.

1. when the value is NULL
2. when the datatype is related to time (datetime, time, ...) and the value is not supported format (ex: '0000-00-00 00:00:00')
3. when the datatype is set and the value is empty

Since we cannot distinguish these cases, we're going to provide detail information for None value.

Changes

• In RowsEvent, we added __none_sources variable.
• When we execute __read_values_name, we check detail information for None cases and save it to __none_sources.
• When dump() executes, we check for none_sources and show them together with column name and value.

Example

• SQL query

  CREATE TABLE test_table (col0 int, col1 varchar(10), col2 datetime);
  INSERT INTO test_table VALUES (1, 'abc', '2023-09-09 00:00:00');
  UPDATE test_table SET col1=NULL, col2='0000-00-00 00:00:00' WHERE col0=1;

• dump result for UPDATE

    === UpdateRowsEvent ===
    Date: 2023-09-09T05:23:14
    Log position: 1360
    Event size: 37
    Read bytes: 13
    Table: test.test_table
    Affected columns: 3
    Changed rows: 1
    Affected columns: 3
    Values:
    --
    *col0:1=>1
  + *col1:abc=>None(null)
  + *col2:2023-09-09 00:00:00=>None(out of datetime range)