Fix empty multipart name (#2750)

libafl/src/inputs/multi.rs

@@ -154,6 +154,7 @@ where
     I: Input,
 {
     fn generate_name(&self, id: Option<CorpusId>) -> String {
+        if self.names().len() > 0 {
             self.names
                 .iter()
                 .cloned()
@@ -161,5 +162,8 @@ where
                 .map(|(name, generated)| format!("{name}-{generated}"))
                 .collect::<Vec<_>>()
                 .join(",")
+        } else {
+            "empty_multipart".to_string() // empty strings cause issues with OnDiskCorpus
+        }
     }
 }