[fix][broker] Extract additional servlets to the default directory by…

[andrasbeni]

… default

Fixes #17472

Motivation

AdditionalServlets.load does not use the default value of narExtractionDirectory when the property is set to an empty string (which is the default in conf/broker.conf).

Modifications

Similar to FieldParser.update, empty string and null are made equivalent to AdditionalServlets.

Verifying this change

• [ ] Make sure that the change passes the CI checks.

This change can be verified as follows:

1. Create an AdditionalServlet implementation.
2. Configure the broker to use it.
3. Leave narExtractionDirectory in the broker.conf the default (empty string)
4. Start the broker.
5. Check /tmp/pulsar-nar for the extracted nar file

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (no)
• The public API: (no)
• The schema: (no)
• The default values of configurations: (no)
• The wire protocol: ( no)
• The rest endpoints: (no)
• The admin cli options: (no)
• Anything that affects deployment: (no)

Documentation

Check the box below or label this PR directly.

Need to update docs?

• [x] doc-not-needed This is a bugfix

[andrasbeni]

/pulsarbot rerun-failure-checks

[andrasbeni]

/pulsarbot run-failure-checks

[andrasbeni]

/pulsarbot run-failure-checks

[andrasbeni]

@codelipenghui , do I need to rebase this PR and test it in my own fork, or can it be merged without all that?