Fail if the key path is empty.

[ioquatix]

Fixes #56.

We could accept nil keys, but there is no way to reasonably format it, e.g. consider {nil => {nil => "foo"}} would format, hypothetically, as []=foo which would decode as {nil => ["foo"]}. I can't see any way to disambiguate the situation.

Types of Changes

• Bug fix.

Contribution

• I added tests for my changes.
• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.