fixes issue #1926

[vsemichev]

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
• [x] 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

• [x] 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.
• [x] All new and existing tests passed.