write_schema modify_field / remove_field on a nonexistent document reports that it "exists"

modify_field (no base_version) or remove_field (base_version="1.1") on a reference that does not exist passes the version checks (a new XWikiDocument reports version 1.1) and then fails in the shared class-fields guard with Document "X" exists but defines no class fields.

Expected: Document "Sandbox.NoSuchClass" does not exist; the first add_field creates the class.

Actual: "exists but defines no class fields".