build_version_file: fix when run within tag-less cset

tools/build_version_file/main.go

@@ -21,8 +21,8 @@
 		}
 		lines = append(lines, l)
 	}
-	if len(lines) != 1 {
+	if len(lines) > 1 {
 		fmt.Println(string(out))
 
 		return "", errors.New("Expects a single line")
 	}
@@ -25,5 +25,8 @@
 		fmt.Println(string(out))
 
 		return "", errors.New("Expects a single line")
 	}
+	if len(lines) == 1 {
+		return lines[0], err
+	}
 
@@ -29,5 +32,5 @@
 
-	return lines[0], err
+	return "", err
 }
 
 func mustSysCmd(name string, arg ...string) string {