sync: fix null handling when updating filepaths/metadata

[ee7]

For now, let's resolve the problem by stripping any optional key with
the value of null when updating filepaths or metadata.

Edit: Okay, I've tried to keep preserving the null during sync (fmt strips it).

---

Before this commit, when updating a .meta/config.json file that
contained an optional key with the value of null, configlet would
produce an UnpackDefect. For example, the below tries to update a
file that contains "contributors": null:

$ git clone https://github.com/exercism/common-lisp
$ cd common-lisp
$ git checkout 4fc524e24a69
$ configlet sync --metadata -e binary -u
Checking exercises...
Cloning https://github.com/exercism/problem-specifications/... success
[warn] metadata: unsynced: binary
sync the above metadata ([y]es/[n]o)? y
options.nim(194)         get
Error: unhandled exception: Can't obtain a value from a `none` [UnpackDefect]

configlet fmt removes optional key/value pairs when the value is
null, and only the Common Lisp and Julia tracks had such a value, but
configlet sync -u should still handle this situation properly.

The bug: our pretty proc serializes from a list of keys, and relied on
optional values being non-null. configlet fmt added only keys with
non-null values to that list, but configlet sync tried to preserve
null values to reduce noise in diffs.

Fix by ensuring that pretty in sync mode can no longer sometimes use
get with a none.

Fixes: #500

[ee7]

I wasn't satisfied with removing the null during sync -u, so I've added a proc that can append a key/null pair instead.

It turns out that this bug would also have affected updating filepaths on the Julia track:

$ find . -path '*.meta/config.json' -exec grep ':\s*null' {} +
./julia/exercises/concept/emoji-times/.meta/config.json:  "forked_from": null
./julia/exercises/concept/encounters/.meta/config.json:  "forked_from": null
./julia/exercises/concept/exercism-matrix/.meta/config.json:  "forked_from": null
./julia/exercises/concept/fibonacci-iterator/.meta/config.json:  "forked_from": null
./julia/exercises/concept/stage-heralding/.meta/config.json:  "forked_from": null