Refuse an empty JMAP id instead of reading it as id 0

An Email/set with mailboxIds {"": true} was accepted and filed the
message in the Inbox. Id::from_str returned 0 for an empty string, and
document 0 is each collection's first: the Inbox for mail. RFC 8620
§1.2 ids are 1 to 255 characters, so "" is refused now, and every
caller already treats a refused id as invalid or not found.

Over-long ids still parse as they did; upstream's test accepts them on
purpose. Found while probing group mailboxes on a scratch server
(specs/multi-account.md, G3).

types tests, jmap_tests and imap_tests pass (RocksDB).

crates/types/src/id.rs

@@ -36,6 +36,13 @@ impl FromStr for Id {
     type Err = ();
 
     fn from_str(s: &str) -> Result<Self, Self::Err> {
+        // inbuxa: an empty id is not id 0. RFC 8620 §1.2 ids are 1 to 255
+        // characters, and "" would otherwise name each collection's first
+        // document: `mailboxIds: {"": true}` filed a message in the Inbox.
+        if s.is_empty() {
+            return Err(());
+        }
+
         let mut id = 0;
 
         for &ch in s.as_bytes() {
@@ -261,4 +268,10 @@ mod tests {
 
         Id::from_str("p333333333333p333333333333").unwrap();
     }
+
+    #[test]
+    fn empty_jmap_id_is_refused() {
+        assert!(Id::from_str("").is_err());
+        assert_eq!(Id::from_str("a").unwrap(), Id::from(0u64));
+    }
 }