Crash when the root primitive is invalid

[sebastienblor]

When exporting from mayaUSD with a root primitive which doesn't exist, we get a crash with a recursive loop.
This happens in a fresh maya scene, by running

cmds.polyCube()
kw = {"exportRoots": ["|pCube1"], "file": "/var/tmp/tmp1uwrns2h.usd", "jobContext": ["Arnold"]}
cmds.mayaUSDExport(**kw)

[autodesk-oss-arnold-bot]

Issue synced internally to ARNOLD-15597