IsInert reports false on empty variant PrimSpecs since v0.21.5

[jonjondev]

Description of Issue

Pre-v0.21.5, SdfSpec::IsInert could be used to check whether a variant brought in any opinions on a variant PrimSpec, where an empty variant would be considered "inert".

Since including required fields as part of SdfLayer::ListFields in commit b7648a6, this particular case will result in IsInert reporting that the variant is no longer inert (False).

@gitamohr requested for this to be filed as an issue on the USD Interest Group.

Steps to Reproduce

With the following file contents:

test.usda

#usda 1.0

over "test"
{
    variantSet "vars" = {
        "off" {
        }
        "render" (
            payload = @...@
        ) {
        }
    }
}

test.py

from pxr import Sdf
layer = Sdf.Layer.FindOrOpen("./test.usda")
layer.GetPrimAtPath("/test{vars=off}").isInert

isInert will report False under USD v0.21.5+, and True for any previous version.

[jilliene]

Filed as internal issue #USD-6960