Exporting with `-exportRoots` flag fails to export prototypes' geometry for Instancers

**Describe the bug**

When exporting a maya instancer node with `-exportRoots instancer_node` the USD file `PointInstancer`'s prototypes in the USD data will have no geometry defined:
```
def "Prototypes" (
kind = "group"
)
{
def "pCube1_0"
{
}
}
```
Versus using e.g. `-selection` flag with instancer node selected:
```
def "Prototypes" (
kind = "group"
)
{
def Mesh "pCube1_0" (
kind = "component"
)
{
uniform bool doubleSided = 1
float3[] extent = [(-0.5, -0.5, -0.5), (0.5, 0.5, 0.5)]
int[] faceVertexCounts = [4, 4, 4, 4, 4, 4]
int[] faceVertexIndices = [0, 1, 3, 2, 2, 3, 5, 4, 4, 5, 7, 6, 6, 7, 1, 0, 1, 7, 5, 3, 6, 0, 2, 4]
point3f[] points = [(-0.5, -0.5, 0.5), (0.5, -0.5, 0.5), (-0.5, 0.5, 0.5), (0.5, 0.5, 0.5), (-0.5, 0.5, -0.5), (0.5, 0.5, -0.5), (-0.5, -0.5, -0.5), (0.5, -0.5, -0.5)]
texCoord2f[] primvars:st = [(0.375, 0), (0.625, 0), (0.375, 0.25), (0.625, 0.25), (0.375, 0.5), (0.625, 0.5), (0.375, 0.75), (0.625, 0.75), (0.375, 1), (0.625, 1), (0.875, 0), (0.875, 0.25), (0.125, 0), (0.125, 0.25)] (
customData = {
dictionary Maya = {
token name = "map1"
}
}
interpolation = "faceVarying"
)
int[] primvars:st:indices = [0, 1, 3, 2, 2, 3, 5, 4, 4, 5, 7, 6, 6, 7, 9, 8, 1, 10, 11, 3, 12, 0, 2, 13]
}
}
```

In this case the instancer node I exported was a top level in the maya scene in both cases

**Steps to reproduce**

Steps to reproduce the behavior:

1. Create a Maya instancer that instances a cube
2. Export `mayaUSDExport` with flag `-exportRoots` set to the instancer node
3. Check the exported USD file -it contains no geometry data.

**Expected behavior**

I'd have expected prototypes to still be exported as intended - but the `-exportRoots` flag only defining the root for the Instancer node itself.

**Attachments**
If applicable, add screenshots, sample files, etc to help explain your problem.

**Specs (if applicable):**
- OS & version: Windows 10
- Maya version: 2024.1
- Maya USD release: 0.25.0
- behavior is the same in: 0.23.1

**Additional context**

The computed `extent` for the instancer when no geometry is included is also infinitesimal, like:
```
float3[] extent = [(3.4028235e38, 3.4028235e38, 3.4028235e38), (-3.4028235e38, -3.4028235e38, -3.4028235e38)]
```
Which I suppose is technically correct due to having no shapes.

Other than that the export appears the same but just misses the prototypes geometry.