Use default value when entity geometry attributes are undefined

[hpinkos]

Fixes #6369

The crash was happening because color was undefined for half the interval. This change uses Property.getValueOrUndefined to make sure we're not trying to set attributes to an undefined value.

Also added specs because we're missing a ton of unit test coverage for entity geometry batches

[cesium-concierge]

@hpinkos, thanks for the pull request! Maintainers, we have a signed CLA from @hpinkos, so you can review this at any time.

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[mramato]

Looks like you have some test failures (possibly epsilon related?)

[mramato]

Otherwise this looks good to me, bump when ready and I'll do a final smokescreen with #6369 data and merge.

[hpinkos]

@mramato ready

[mramato]

Thanks again.