chore(tvix/eval): clippy warn is length zero

This CL address clippy warning about finding the zero
length of something using `is_empty()` instead of `len() == 0`.

Change-Id: I2b36c7c7b65b733609fc0dcd33be06f9d772bc9b
Signed-off-by: Aaqa Ishtyaq <aaqaishtyaq@gmail.com>
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8029
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/value/mod.rs

@@ -553,7 +553,7 @@ fn total_fmt_float<F: std::fmt::Write>(num: f64, mut f: F) -> std::fmt::Result {
         }
 
         // if we modified the scientific notation, flip the reference
-        if new_s.len() != 0 {
+        if !new_s.is_empty() {
             s = &mut new_s
         }
     }