fix(update_child_repos.sh): add file existence check for file processing.

- if file does not exist do NOT continue (hence process this file)
- three way merging still works as before

update_child_repos.sh

@@ -88,7 +88,7 @@ while IFS= read -r clone_url; do
     conflicted_files=()
 
     for file in ${FILES_TO_UPDATE}; do
-      if ! is_file_changed "${file}"; then
+      if ! is_file_changed "${file}" && [ -f "${file}" ]; then
         continue
       fi