Improve validation error for empty mxid.

Previously an empty MXID was reported as MissingLeadingSigil but a more
concise variant is just Empty.

Signed-off-by: Jason Volk <jason@zemos.net>

crates/ruma-identifiers-validation/src/lib.rs

@@ -22,17 +22,17 @@ pub use error::Error;
 const MAX_BYTES: usize = 255;
 
 /// Checks if an identifier is valid.
-fn validate_id(id: &str, first_byte: u8) -> Result<(), Error> {
+fn validate_id(id: &str, sigil: u8) -> Result<(), Error> {
     #[cfg(not(feature = "compat-arbitrary-length-ids"))]
     if id.len() > MAX_BYTES {
         return Err(Error::MaximumLengthExceeded);
     }
 
-    if id.as_bytes().first() != Some(&first_byte) {
-        return Err(Error::MissingLeadingSigil);
+    match id.as_bytes().first() {
+        Some(&first_byte) if first_byte == sigil => Ok(()),
+        Some(_) => Err(Error::MissingLeadingSigil),
+        None => Err(Error::Empty),
     }
-
-    Ok(())
 }
 
 /// Checks an identifier that contains a localpart and hostname for validity.