[ProgressBar] Update progressPerc to be null, not undefined

[jvigliotta]

Is your feature request related to a problem? Please describe.
Our ProgressBar component https://github.com/nasa/openmct/blob/master/src/ui/components/ProgressBar.vue currently uses undefined for the percentage of progress property of the model. This should most likely be null as pointed out here: #6067 (comment)

Describe the solution you'd like
Update to check for null. Update any other uses of ProgressBar in the code bass and update to pass in null.

Describe alternatives you've considered
Leaving as is, but this is less intuitive to what it actually represents.

[jvigliotta]

@ozyx what if I wanted that extrra "r"?