fix: a request whose origin has no host is refused, not treated as a site (closes #527)

A page with an opaque origin, such as one its server sandboxes with a
Content-Security-Policy, is reported to the background with the origin
"null", and Chrome reports a local file as "file://". The background took
either as a site, so every such page was one site: a connection remembered
for one covered them all, and the phishing check never matched. The
background now parses the reported origin and, when it has no host, refuses
the request with 4100 like one with no origin, before any method runs.
tests/rpcOrigin.test.js sends both, and README's SiteApproval says what is
refused.

Model: opus-5-5

README.md

@@ -2129,14 +2129,19 @@ view would leave a wallet one click from deletion.
   `wallet_requestPermissions` and is on neither the allowed nor the denied list.
   A site is its full origin, `scheme://host[:port]`, on both lists and for a
   connection allowed without "Remember": a choice for `https://dapp.example`
-  says nothing about `http://dapp.example` or another port of that host. The
-  background script prefers the toolbar popup (`action.openPopup()`) and falls
-  back to a separate popup window (`src/background/index.js`,
-  `requestApproval()`). Only one exists per site at a time: a further connection
-  request from a site whose prompt is still unanswered is refused with EIP-1193
-  code `-32002` and opens no new prompt. If that prompt was in a toolbar popup
-  that closed before it connected, and the toolbar popup has since been set to
-  open something else, the refused request shows that prompt again.
+  says nothing about `http://dapp.example` or another port of that host. A
+  request whose origin has no host names no site, so it is refused with EIP-1193
+  code `4100` and opens no prompt. Two such origins are `null`, which the
+  browser reports for a page with an opaque origin, such as one its server
+  sandboxes with a Content-Security-Policy, and `file://`, which Chrome reports
+  for a local file. The background script prefers the toolbar popup
+  (`action.openPopup()`) and falls back to a separate popup window
+  (`src/background/index.js`, `requestApproval()`). Only one exists per site at
+  a time: a further connection request from a site whose prompt is still
+  unanswered is refused with EIP-1193 code `-32002` and opens no new prompt. If
+  that prompt was in a toolbar popup that closed before it connected, and the
+  toolbar popup has since been set to open something else, the refused request
+  shows that prompt again.
 - **Elements**:
     - "Connection Request" heading
     - Phishing warning banner (shown when the hostname is on the phishing