Can't correlate report entries for zero-valued structs when producing MachineConfigs

[bgilbert]

This config has an invalid Filesystem object that parses into a zero-valued struct:

variant: openshift
version: 4.8.0-experimental
metadata:
  name: z
storage:
  filesystems:
    - z: q

When transpiling with --raw FCCT produces the correct line/column information:

warning at $.storage.filesystems.0.z, line 7 col 7: Unused key z
error at $.storage.filesystems.0.device, line 7 col 7: path not specified
Error translating config: config generated was invalid

But when transpiling to a MachineConfig it does not:

warning at $.storage.filesystems.0.z, line 7 col 7: Unused key z
error at $.spec.config.storage.filesystems.0.device, line 1 col 1: path not specified
Error translating config: config generated was invalid

The reason is that both the Ignition merge transcript and the FCCT TranslationSet omit translation info for zero-valued structs, to keep the translation compact (for debugging and test case writing; this doesn't affect normal use). When we're producing an Ignition config and thus not reparenting the output struct, translateReportPaths() falls back to leaving the input path unchanged, that path exists in the TranslationSet, and Correlate() is able to set the report entry marker correctly. But when we're producing a MachineConfig, the report entry path doesn't exist in the TranslationSet, and the nearest available ancestor of the bad line is the root of the input struct at line 1, column 1.