Support for using `--set` for array as value

As described in the documentation for the --set option, individual elements can be added to an array like this:

$ sops --set '["an_array"][1] "secretuser2"' ~/git/svc/sops/example.yaml

As a shortcut, it would be nice if it were also possible for a full JSON array to be specified as a value. When I try to do this, though, the value set into the encrypted file is null. For example:

$ sops --set '["an_array"] ["element"]' test.yaml

$ sops -d test.yaml
an_array: null

Note that I can set an array as the value for a property on a map:

$ sops --set '["arr"] {"nested": ["element"]}' test.yaml

$ sops -d test.yaml
an_array:
    nested:
        - element

Note that I was using version 3.7.3 for these tests.

Start with the CLI's --set option and reproduce the documented array-value example using sops 3.7.3. Trace why a full JSON array becomes null while an array nested in a map works, then add coverage for direct array values and verify decryption preserves the elements.