Added undefined protection to slider javascript

[jmbarne3]

Description
A bit of data that is supposed to be added to the month and year links is not being added when the page is rendered. I have not been able to track down why the data attribute is not being added, but the process is supposed to fail gracefully and currently is not. I've added logic that allows the fallback logic - of linking to that calendar month or year view, respectively - to run if the data-ajax-link isn't defined.

Motivation and Context
This will correct the bug reported by Roger in the events channel.

How Has This Been Tested?
Changes are in dev and qa.

Types of changes

• 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 an update to the documentation.
• I have updated the documentation accordingly.