extra include under all when using -o

[rsgalloway]

a stray "include: []" gets added under "all" when creating blank .env files:

$ envstack asdf -o asdf.env
$ cat asdf.env
#!/usr/bin/env envstack
include: []
all: &all
  <<: *all
  include: []
darwin:
  <<: *all
linux:
  <<: *all
windows:
  <<: *all

looks like this is happening in partition_platform_data when it copies all the data to be partitioned under the "all" key. I think this was done for exporting encrypted files, but I'm not sure. a potential fix is to set it to an empty dict.