fix(stamp): validate image filename only for image stamp type (#4099)

# Description of Changes

- **What was changed**: Moved the filename validation logic for
`stampImage` inside a condition that checks whether the stamp type is
`"image"`.
- **Why the change was made**: Previously, the validation was applied
regardless of stamp type, leading to unnecessary errors for
non-image-based stamps where no `stampImage` is provided.

Closes #4097

---

## 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/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

---------

Co-authored-by: Copilot <[email protected]>

app/core/src/main/java/stirling/software/SPDF/controller/api/misc/StampController.java

@@ -70,10 +70,15 @@ public class StampController {
         String stampType = request.getStampType();
         String stampText = request.getStampText();
         MultipartFile stampImage = request.getStampImage();
+        if ("image".equalsIgnoreCase(stampType)) {
+            if (stampImage == null) {
+                throw new IllegalArgumentException("Stamp image file must be provided when stamp type is 'image'");
+            }
             String stampImageName = stampImage.getOriginalFilename();
-        if (stampImageName.contains("..") || stampImageName.startsWith("/")) {
+            if (stampImageName == null || stampImageName.contains("..") || stampImageName.startsWith("/")) {
                 throw new IllegalArgumentException("Invalid stamp image file path");
             }
+        }
         String alphabet = request.getAlphabet();
         float fontSize = request.getFontSize();
         float rotation = request.getRotation();