fix(resolver): do not panic when sorting empty summaries

Author: weihanglo

src/cargo/core/resolver/version_prefs.rs

@@ -125,7 +125,7 @@ impl VersionPreferences {
                 VersionOrdering::MinimumVersionsFirst => cmp,
             }
         });
-        if first_version.is_some() {
+        if first_version.is_some() && !summaries.is_empty() {
             let _ = summaries.split_off(1);
         }
     }
@@ -284,12 +284,12 @@ mod test {
         );
     }
 
-    #[should_panic = "`at` split index (is 1) should be <= len (is 0)"]
     #[test]
     fn test_empty_summaries() {
         let vp = VersionPreferences::default();
         let mut summaries = vec![];
 
         vp.sort_summaries(&mut summaries, Some(VersionOrdering::MaximumVersionsFirst));
+        assert_eq!(summaries, vec![]);
     }
 }