Export does not produce empty element for empty inputs

Continuing from #2802

Exporting a configuration that has no value for some of its inputs does not produce an empty element in the exported content.

Consider the following `folder-auth` model for example:
```
jenkins:
authorizationStrategy:
folderBased:
folderRoles:
- folders:
- "xxx"
name: "xxx"
permissions:
- id: "Job/Build"
- id: "Job/Discover"
- id: "Job/Read"
sids: []
```
The export configuration page does not display the empty "sids" element.

It seems `2053.vb_0da_47381a_25` introduced `@ParametersAreNonnullByDefault` which caused problems with my CASC configuration without "sids" element from initial export.

`folder-auth` is just one such case I had the misfortune of having to deal with, but there could be many others with similar symptoms.