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Description
The protocol indicates that the response to the textDocument/diagnostic request is a proper DiagnosticReport, yet certain language servers return null as a response (like the Ruby language server).
Initially one would blame this on the misbehaving server that's not following the specification, yet one finds editors/clients that accommodate this behavior (example here).
Hence the issue is: Should we allow null responses for this request and send the same reply as VS Code does? Or should we handle this as an invalid reply?