MissingDataChangeGenerator: small optimisation, skip null-value columns (issues #6420)

[catull]

Export table data preserving NULL-values, or not.

Impact

• Enhancement/New feature (adds functionality without impacting existing logic)

Description

Have you ever wondered if changelog exports could skip all NULL-values ?
This MR does exactly this.

It implements issue #6420.

[catull]

Hey @catull,

Could you please review the changes made on the serializer, I think they are not yet fixing these cases. For example what has been added in XMLChangeLogSerializer:

if (value instanceof ColumnConfig) {
      ColumnConfig columnConfig = (ColumnConfig) value;
      if (!preserveNullValues && columnConfig.isNull()) {
          return;
      }
      node.appendChild(createNode((LiquibaseSerializable) value));
}

is broken the below assertion from serialize_pretty_nestedNodeWithAttributesAndNotExistsIgnored() (XMLChangeLogSerializerTest)

assertEquals("<createTable catalogName=\"a\"\n" +
                "        schemaName=\"b\"\n" +
                "        tableName=\"c\">\n" +
                "    <column defaultValue=\"x1\" name=\"x\"/>\n" +
                "    <column defaultValue=\"y1\" name=\"y\"/>\n" +
                "</createTable>", out);

Because we are not including defaultValue in the node as it was behaving previously. Additionally, it would be worth checking other serializers to see if the same issue is happening. At the moment, we don't have any reported issues for those serializers related to this change, but that's because we don't have specific tests for them.

Thanks, Daniel.

I adapted the code to only emit value="null" where necessary.
Please review it, thank you.

[catull]

@MalloD12 Can you please review the changes ? Thanks.