Handle empty branch in convert

`convert` previously returned a misleading "Incomplete list" error
when called on an empty `node`, because the indices branch ran with
`minValue == math.MaxInt`. Short-circuit the all-empty case to
return an empty `sops.TreeBranch` instead, which is the natural
identity for `unflattenTreeBranch(sops.TreeBranch{})`.

Signed-off-by: Hidde Beydals <hidde@hhh.computer>

Author: hiddeco

stores/flatten.go

@@ -122,6 +122,9 @@ func convert(root *node) (interface{}, error) {
 	if b2i(hasValue)+b2i(hasSubkey)+b2i(hasIndex) > 1 {
 		return nil, fmt.Errorf("Type mismatch")
 	}
+	if !hasValue && !hasSubkey && !hasIndex {
+		return sops.TreeBranch{}, nil
+	}
 	if hasValue {
 		return *root.value, nil
 	}

stores/flatten_test.go

@@ -202,6 +202,10 @@ func TestUnflattenTreeBranch(t *testing.T) {
 	assert.NotNil(t, err)
 	assert.Equal(t, "Error while unflattening: Type mismatch", err.Error())
 	assert.Nil(t, output)
+
+	output, err = unflattenTreeBranch(sops.TreeBranch{})
+	assert.Nil(t, err)
+	assert.Equal(t, sops.TreeBranch{}, output)
 }
 
 func TestFlattenTreeBranch(t *testing.T) {