Make inputs optional if the defaults are set empty

[JuhaKiili]

Users want to define optional inputs with valohai-utils, but currently, it is not supported.

You define an input with valohai-utils like this:

default_inputs = {
  "my_input": "https://herpderp.com/lol.txt"
}

Generated YAML:

- name: my_input
  default:
  - https://herpderp.com/lol.txt
  optional: false

This PR introduces support for optional inputs. Just leave the value empty:

default_inputs = {
  "my_input": ""
}

Generated YAML:

- name: my_input
  optional: true

[tokkoro]

💯