fix: correct y axis title in boxplot when mark.extent isnt set

[MarcBaeuerle]

closes #9223

The issue rose when mark.extent was not explicitly set, causing the tukey box layer encoding to have a missing axis object. The original implementation removes the axis.title despite it being different from the title when explicitly setting the axis.title. In the issue, the title itself was 'shannon' while the axis.title was 'new title'.

Checklist

• This PR is atomic (i.e., it fixes one issue at a time).
• The title is a concise semantic commit message (e.g. "fix: correctly handle undefined properties").
• yarn test runs successfully

[MarcBaeuerle]

Hi @domoritz, I don't mean to be rude by pinging you, but I just included a pr description to give some context into the changes I made. Let me know if there is anything else missing since I'm new to this. Thank you!

[domoritz]

Thanks for the pull request. I'm super overloaded with end of semester things right now. Hopefully someone else can look at this or I will over the winter break.