fix: display title when set in config.axisX/axisY.title

[MarcBaeuerle]

potentially closes #9429

This fixes the compilation process from vega lite to vega by accounting for the title property inside the config.
@domoritz suggested removing support for setting the axis title inside the config entirely, but due to the consensus in #4057 I chose this solution instead.

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