Add safety check for package list validation

pkg/merge/merge.go

@@ -1,9 +1,16 @@
 package merge
 
-func Merge(packages map[string]bool) []string {
+import "errors"
+
+var ErrEmptyList = errors.New("package list is empty")
+
+func Merge(packages map[string]bool) ([]string, error) {
 	result := make([]string, 0, len(packages))
 	for name := range packages {
 		result = append(result, name)
 	}
-	return result
+	if len(result) == 0 {
+		return nil, ErrEmptyList
+	}
+	return result, nil
 }