fix(lsp): decode 'null' in server responses as vim.NIL #34849

Problem:
Previously, 'null' value in LSP responses were decoded as 'nil'.
This caused ambiguity for fields typed as '? | null' and led to
loss of explicit 'null' values, particularly in 'data' parameters.

Solution:
Decode all JSON 'null' values as 'vim.NIL' and adjust handling
where needed. This better aligns with the LSP specification,
where 'null' and absent fields are distinct, and 'null' should
not be used to represent missing values.

This also enables proper validation of response messages to
ensure that exactly one of 'result' or 'error' is present, as
required by the JSON-RPC specification.

runtime/doc/news.txt

@@ -86,6 +86,7 @@ LSP
   arguments corresponding to a log entry instead of the individual arguments.
 • `vim.lsp.semantic_tokens.start/stop` now renamed to
   `vim.lsp.semantic_tokens.enable`
+• Missing fields in LSP messages are now represented using |vim.NIL| instead of nil.
 
 LUA