Corrects missing secondary Y-axis title

Description

Corrects the line in the drawing.go file that set secondary axis title to nil.

Related Issue

#1926

Motivation and Context

I need this fix for one of my projects where I need to set the secondary Y-axis title

How Has This Been Tested

Ran go test which came out clean. Performed two tests using my project binary - one to create a combo column + line chart with a secondary Y-axis that has a title and another one that created a combo column + line chart with invisible secondary Y-axis. Both tests produced expected results.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.