Fixed bug with exporting to CSV when categories are null

[yungfox]

New Pull Request

I encountered a bug while attempting to export chart data to CSV from a line chart that had null datetime categories and values (to show inconsistencies of data over time), and I was getting the same error as described on #2101, #2185 and #3247. My fix simply checks if the current category is null and moves on to the next if such is the case.

Fixes #2101, #2185 and #3247.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes