bug#66144: 29.1; eglot-shutdown request params violate JSONRPC spec

[Aaron Zeng]

When using Eglot with ocamllsp (or other language servers built with the
same OCaml LSP server library), M-x eglot-shutdown sends an invalid
JSONRPC request to the language server and causes it to exit with an
error message instead of gracefully.  As an additional side effect of
the error, eglot will restart/reconnect to the language server instead
of staying disconnected.

1. emacs -Q
2. C-x C-f ~/temp.ml
3. M-: (setq major-mode 'tuareg-mode)
4. M-x eglot
5. C-u M-x eglot-shutdown

This produces the following *Messages*:

    [eglot] Connected! Server `ocamllsp' now managing `(tuareg-mode caml-mode 
reason-mode)' buffers in project `azeng'.
    [eglot] Asking EGLOT (azeng/(tuareg-mode caml-mode reason-mode)) politely 
to terminate
    [jsonrpc] Server exited with status 1
    jsonrpc-error: "request id=5 failed:", (jsonrpc-error-code . -1), 
(jsonrpc-error-message . "Server died"), (jsonrpc-error-data)

And from the events buffer:

    [client-request] (id:2) Thu Sep 21 16:48:11 2023:
    (:jsonrpc "2.0" :id 2 :method "shutdown" :params nil)
    [stderr] (monitor.ml.Error
    [stderr]  ("Jsonrpc: json conversion failed: invalid structured value")
    [stderr]  ("Raised at Lsp_json_rpc_types__Import.Json.error in file 
\"import.ml\" (inlined), line 31, characters 23-50"
    [stderr]   "Called from Lsp_json_rpc_types__Jsonrpc.Structured.t_of_yojson 
in file \"jsonrpc.ml\" (inlined), line 59, characters 14-56"
    [stderr]   "Called from Lsp_json_rpc_types__Import.Option.map in file 
\"import.ml\" (inlined), line 7, characters 21-26"
    [stderr]   "Called from Lsp_json_rpc_types__Import.Json.field in file 
\"import.ml\" (inlined), line 46, characters 31-79"
    [stderr]   "Called from Lsp_json_rpc_types__Jsonrpc.Packet.t_of_fields in 
file \"jsonrpc.ml\", line 298, characters 21-77"
    [stderr]   "Called from Lsp__Io.Make.read in file \"io.ml\" (inlined), line 
108, characters 24-57"
    [stderr]   "Called from Fiber__Core.O.(>>=).(fun) in file \"core.ml\", line 
251, characters 34-39"
    [stderr]   "Called from Fiber__Scheduler.exec in file \"scheduler.ml\", 
line 76, characters 8-11"
    [stderr]   "Re-raised at Stdune__Exn.raise_with_backtrace.(partial) in file 
\"exn.ml\" (inlined), line 38, characters 27-56"
    [stderr]   "Called from Stdune__Exn_with_backtrace.reraise in file 
\"exn_with_backtrace.ml\" (inlined), line 20, characters 33-71"
    [stderr]   "Called from Fiber__Scheduler.exec in file \"scheduler.ml\", 
line 81, characters 30-60"
    [stderr]   "Called from Fiber__Scheduler.advance in file \"scheduler.ml\" 
(inlined), line 194, characters 2-58"
    [stderr]   "Called from Fiber_async.deferred_of_fiber.loop.(fun) in file 
\"fiber_async.ml\" (inlined), line 44, characters 11-53"))
    [internal] Thu Sep 21 16:48:11 2023:
    (:message "Connection state changed" :change "exited abnormally with code 
1\n")
    
    ----------b---y---e---b---y---e----------

It seems that jsonrpc.el is sending a "null" value for the params field
of a request, when it should instead be omitting the field or sending an
empty JSON object instead:
https://www.jsonrpc.org/specification#parameter_structures

Users of other editors have reported similar issues, and concluded that
this was correct (if pedantic) behavior by ocamllsp and should be fixed
in the editor: https://github.com/helix-editor/helix/issues/5400


In GNU Emacs 29.1 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.15.12, Xaw scroll bars) of 2023-09-21 built on
 igm-qws-u12685a
Repository revision: a9b28224af0f73d1fe0f422e9b318c5b91af889b
Repository branch: HEAD
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Rocky Linux 8.8 (Green Obsidian)

Configured using:
 'configure --with-x-toolkit=lucid --without-gpm --without-gconf
 --without-selinux --without-imagemagick --with-modules --with-gif=no
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Configured features:
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ZLIB

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Major mode: Fundamental

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Load-path shadows:
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Memory information:
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