Fix scrollbar errors when offset is zero

[pradeepnschrodinger]

Description

This fixes the issue seen at #723. The onmousedown handler for Scrollbar throws an error when the user tries to scroll at the beginning of the scroll content area, which happens because targetTouches is undefined at: https://github.com/schrodinger/fixed-data-table-2/blob/522e82be907c41c250b231a76e3127abed9aed81/src/plugins/Scrollbar.js#L527-L537 because we're actually doing a normal mouse click instead of a "touch" click. But the code shouldn't have reached here in the first place.

The root problem is that FDT thinks it's a touch event because of the way we use the || operator in https://github.com/schrodinger/fixed-data-table-2/blob/522e82be907c41c250b231a76e3127abed9aed81/src/plugins/Scrollbar.js#L358-L364 We should instead use the ?? operator for cases like this.

Motivation and Context

Fixes #723

How Has This Been Tested?

Tested on local examples

Types of changes

• [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.
• [x] I have read the CONTRIBUTING document.

[pradeepnschrodinger]

@karry08 , friendly ping

[karry08]

lgtm

[pradeepnschrodinger]

Released with v2.0.12.