internal/mcp: avoid JSON nulls on the wire

Add a test that checks for JSON null on the wire, and fix two categories
of bugs leading to null:

1. In many places, we could pass typed nil where we shouldn't have. Add
   an orZero helper to avoid typed nills.
2. When listing roots, we should avoid nil.

Fixes golang/go#74105

Change-Id: I061420487dbfcbf9016cb94b39bd72270e871c0e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/682675
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

mcp/testdata/conformance/server/resources.txtar

@@ -101,8 +101,7 @@ info.txt
 {
 	"jsonrpc": "2.0",
 	"id": 1,
-	"method": "roots/list",
-	"params": null
+	"method": "roots/list"
 }
 {
 	"jsonrpc": "2.0",