rust: Exclude empty node prefixes

We presumably don’t want `--rev ""` to select every single revision,
even though the empty string is a prefix of all strings.

Differential Revision: https://phab.mercurial-scm.org/D9862

rust/hg-core/src/revlog/node.rs

@@ -160,7 +160,7 b' impl NodePrefix {'
     pub fn from_hex(hex: impl AsRef<[u8]>) -> Result<Self, FromHexError> {
         let hex = hex.as_ref();
         let len = hex.len();
-        if len > NODE_NYBBLES_LENGTH {
+        if len > NODE_NYBBLES_LENGTH || len == 0 {
             return Err(FromHexError);
         }
 
@@ -201,10 +201,6 b" impl<'a> NodePrefixRef<'a> {"
         }
     }
 
-    pub fn is_empty(&self) -> bool {
-        self.len() == 0
-    }
-
     pub fn is_prefix_of(&self, node: &Node) -> bool {
         if self.is_odd {
             let buf = self.buf;