annotate: remove redundant .is_absent() test from get_file_contents()

Here we shouldn't care whether the file value is absent or a tree, for example.

lib/src/annotate.rs

@@ -251,9 +251,6 @@ fn get_file_contents(
     tree: &MergedTree,
 ) -> Result<Vec<u8>, BackendError> {
     let file_value = tree.path_value(path)?;
-    if file_value.is_absent() {
-        Ok(Vec::new())
-    } else {
     let effective_file_value = materialize_tree_value(store, path, file_value).block_on()?;
     match effective_file_value {
         MaterializedTreeValue::File { mut reader, id, .. } => {
@@ -280,5 +277,4 @@ fn get_file_contents(
         }
         _ => Ok(Vec::new()),
     }
-    }
 }