fix(tvix/eval): len_without_is_empty clippy warn

This CL addresses clippy warning len_without_is_empty
which expects `.is_empty()` method to be present when
implementing `.len()` method for an item.

Change-Id: I8878db630b9ef5853649a906b764a33299bb5dc8
Signed-off-by: Aaqa Ishtyaq <aaqaishtyaq@gmail.com>
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7806
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/mod.rs

@@ -898,7 +898,7 @@ mod pure_builtins {
     fn builtin_tail(_: &mut VM, list: Value) -> Result<Value, ErrorKind> {
         let xs = list.to_list()?;
 
-        if xs.len() == 0 {
+        if xs.is_empty() {
             Err(ErrorKind::TailEmptyList)
         } else {
             let output = xs.into_iter().skip(1).collect::<Vec<_>>();