Fix "Cannot read property 'slice' of undefined" bug with min/max dates

[travellingprog]

Description

This fixes a bug in makeDateValues() that occurs if you specified a min and/or max date, but have set the value of the control to null or an empty value. It was caused by some typos.

You can reproduce this issue by set minDate (and/or maxDate), clicking to change the date, and then deleting the value.

Motivation and Context

This fixes a bug that causes a runtime error that prevents makeDateValues() from fully running. That has other implications, including the onChange handler not running.

In the browser console, you'll get the error message Uncaught TypeError: Cannot read property 'slice' of undefined, as reported in issue #134

How Has This Been Tested?

Only manually.

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 a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[JonShort]

Good work 👍