[Fix] Handle missing end page in PDF split range (#2816)

## Summary of Changes

**What was changed:**
- Updated the `handlePart` method to handle cases where the end page is
not specified (e.g., '1-').
- The method now defaults to the last page of the PDF, improving the
feature's usability.

**Why the change was made:**
- Users often forget the total page count when splitting PDFs. The new
feature ensures that when only the starting page is specified, the range
defaults to the last page, preventing errors like
`ArrayIndexOutOfBoundsException`.

**Any challenges encountered:**
- No significant challenges encountered while implementing this feature.

**Closes #1576**

---

## Checklist

### General
- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings


### Testing
- [x] I have tested my changes locally.

src/main/java/stirling/software/SPDF/utils/GeneralUtils.java

@@ -285,7 +285,10 @@ public class GeneralUtils {
             String[] rangeParts = part.split("-");
             try {
                 int start = Integer.parseInt(rangeParts[0]);
-                int end = Integer.parseInt(rangeParts[1]);
+                int end =
+                        (rangeParts.length > 1 && !rangeParts[1].isEmpty())
+                                ? Integer.parseInt(rangeParts[1])
+                                : totalPages;
                 for (int i = start; i <= end; i++) {
                     if (i >= 1 && i <= totalPages) {
                         partResult.add(i - 1 + offset);