Export does not produce empty element for empty inputs

[eliasbalasis]

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.

[timja]

Possibly, but this should really be fixed in the plugin.

Creating a set first and adding the values to it or a null check and creating an empty set would both work