fix: bootstrap apps without version config

scripts/bump.sh

@@ -2,6 +2,7 @@
 set -euo pipefail
 
 : "${INPUT_CONFIG_FILE:?config-file is required}"
+: "${INPUT_VERSION_FILE:=}"
 : "${INPUT_BASE_BRANCH:?base-branch is required}"
 : "${INPUT_TARGET_BRANCH:?target-branch is required}"
 : "${INPUT_BUMP:?bump is required}"
@@ -45,7 +46,11 @@ git fetch "${INPUT_REMOTE}" \
 script_dir=$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)
 base_ref="refs/remotes/${INPUT_REMOTE}/${INPUT_BASE_BRANCH}"
 current_version=$(python3 "${script_dir}/read_version.py" "${INPUT_CONFIG_FILE}")
-base_version=$(python3 "${script_dir}/read_version.py" "${INPUT_CONFIG_FILE}" --ref "${base_ref}")
+base_version_args=("${INPUT_CONFIG_FILE}" --ref "${base_ref}")
+if [[ -n "${INPUT_VERSION_FILE}" ]]; then
+	base_version_args+=(--fallback-version-file "${INPUT_VERSION_FILE}")
+fi
+base_version=$(python3 "${script_dir}/read_version.py" "${base_version_args[@]}")
 
 comparison=$(python3 - "${current_version}" "${base_version}" <<'PY'
 import re