`splice`in DefineListt removing `undefined` values in source array

[mitch-mitchel]

How often can you reproduce it?

• [x] Always
• [ ] Sometimes
• [ ] Rarely
• [ ] Unable
• [ ] I didn’t try

Description: If the source array for a new DefineList contains only values of undefined at each index, it passes the allSame test here when calling splice: https://github.com/canjs/can-define/blob/ea827f80dbac95dceb55384cc9730566d4b488f7/list/list.js#L294-L297

as it thinks nothing will change, and ends up returning a DefineList of length 0. As long as at least one element in the source array is not undefined it works as expected.

var undefinedList = new DefineList([undefined, undefined])
console.log('so much empty', undefinedList.length) //=> 0

See this codepen example: https://codepen.io/mikemitchel/pen/eYOMwmE

Expected results: I expected in the first example of the codepen to have a DefineList of length 2, with the value at each index being undefined.

Environment:

Software	Version
can-define version	"^2.7.10"
Browser	Chrome Version 76.0.3809.132 (Official Build) (64-bit)
Operating system	MacOS 10.14.6 (18G95)