Allow requests to be serialized as nothing instead of an empty object

[swallez]

Allow requests that have all-optional properties to be serialized as an empty body (zero bytes) instead of an empty object ({})

Some endpoints (e.g. /_validate/query) require this behavior and reject requests that have an empty object body.

This is achieved by wrapping the body accessor in endpoint definitions using EndpointBase.nonEmptyJsonObject(bodyGetter). This will be added to the relevant endpoints in the code generator.

[l-trotta]

fixes #924

[github-actions]

The backport to 8.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-8.x 8.x
# Navigate to the new working tree
cd .worktrees/backport-8.x
# Create a new branch
git switch --create backport-926-to-8.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 d78f01e99bbc3048f2d0a5e68165e30f1163352c
# Push it to GitHub
git push --set-upstream origin backport-926-to-8.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.x

Then, create a pull request where the base branch is 8.x and the compare/head branch is backport-926-to-8.x.