* bugfix: fix representation of null in spack_yaml output
Nulls were previously printed differently by `spack config blame config`
and `spack config get config`. Fix this in the `spack_yaml` dumpers.
* bugfix: `spack config blame` should print all lines of config
`spack config blame` was not printing all lines of configuration because
there were no annotations for empty lines in the YAML dump output. Fix
this by removing empty lines.
19b6d3589a