fix(vim.json): properly treat luanil options as booleans (#28622)

Note: Upstream doesn't have this. It's an Nvim addition.

test/functional/lua/json_spec.lua

@@ -32,6 +32,18 @@ describe('vim.json.decode()', function()
       baz = vim.NIL,
       foo = { a = 'b' },
     }, exec_lua([[return vim.json.decode(..., {})]], jsonstr))
+    eq(
+      {
+        arr = { 1, 2, vim.NIL },
+        bar = { 3, 7 },
+        baz = vim.NIL,
+        foo = { a = 'b' },
+      },
+      exec_lua(
+        [[return vim.json.decode(..., { luanil = { array = false, object = false } })]],
+        jsonstr
+      )
+    )
     eq({
       arr = { 1, 2, vim.NIL },
       bar = { 3, 7 },