Document (allow_empty)

[ulugbekna]

Desired Behavior

I recently had the error below suggesting to use (allow_empty) but I couldn't find documentation for this stanza

Error: The package vscode-ocaml-platform does not have any user defined
stanzas attached to it. If this is intentional, add (allow_empty) to the
package definition in the dune-project file
-> required by _build/default/vscode-ocaml-platform.install
-> required by alias all

[Alizter]

Also reported by:

• No error is raised when building an empty package not defined in the dune-project file #6099