fix(bumper): fix unbound variable on new repo 2 (#32)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Bug Fixes**
- Improved the accuracy and consistency of status messages related to
pull requests and pushes.
  
- **New Features**
- Introduced a new function for better retrieval of head labels in
status messages.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

lib/message.sh

@@ -32,8 +32,8 @@ make_push_status() {
   COMPARE=""
   ADDITIONAL_INFO=""
 
-  if [[ -n "${BUMPER_CURRENT_VERSION}" ]]; then
-    COMPARE="**Changes**: [${BUMPER_CURRENT_VERSION}...${BUMPER_NEXT_VERSION}](${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}/compare/${BUMPER_CURRENT_VERSION}...${BUMPER_NEXT_VERSION})"
+  if [[ -n "${BUMPER_CURRENT_VERSION:-}" ]]; then
+    COMPARE="**Changes**: [${BUMPER_CURRENT_VERSION:-}...${BUMPER_NEXT_VERSION}](${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}/compare/${BUMPER_CURRENT_VERSION:-}...${BUMPER_NEXT_VERSION})"
   fi
 
   if [[ -n "${INPUT_BUMP_SEMVER}" && "${INPUT_BUMP_SEMVER}" == "true" ]]; then