Fix cert-sign API NullPointerException when pageNumber is omitted for invisible signatures (#3463)

# Description of Changes

Please provide a summary of the changes, including:

- **What was changed**  
- Updated `SignPDFWithCertRequest` to use `Boolean` for `showSignature`
and `showLogo`, and made `pageNumber` nullable.
  - In `CertSignController`:
- Added an `@InitBinder` to convert empty multipart fields to `null`.
- Extended `@PostMapping` to consume both `multipart/form-data` and
`application/x-www-form-urlencoded`.
- Wrapped `pageNumber` calculation in a null-check (`pageNumber =
request.getPageNumber() != null ? request.getPageNumber() - 1 : null`).
- Changed signature-visualization and logo checks to
`Boolean.TRUE.equals(...)` to avoid unboxing NPE.
  - Cleaned up imports and schema annotations in the request model.

- **Why the change was made**  
- Prevent a 500 Internal Server Error caused by calling `.intValue()` on
a null `pageNumber` when `showSignature=false` (invisible signatures).
- Ensure that omitting `pageNumber` doesn’t break clients using the “try
it out” swagger UI or `curl`-based requests.

- **Any challenges encountered**  
- Configuring Spring’s data binder to treat empty file inputs as `null`
required a custom `PropertyEditorSupport`.
- Balancing backward compatibility with stricter type handling
(switching from primitive `boolean` to boxed `Boolean`).

Closes #3459

---

## 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)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/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/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/DeveloperGuide.md#6-testing)
for more details.

src/main/java/stirling/software/SPDF/model/api/security/SignPDFWithCertRequest.java

@@ -20,7 +20,8 @@ public class SignPDFWithCertRequest extends PDFFile {
 
     @Schema(
             description =
-                    "The private key for the digital certificate (required for PEM type certificates)")
+                    "The private key for the digital certificate (required for PEM type"
+                            + " certificates)")
     private MultipartFile privateKeyFile;
 
     @Schema(description = "The digital certificate (required for PEM type certificates)")
@@ -32,11 +33,11 @@ public class SignPDFWithCertRequest extends PDFFile {
     @Schema(description = "The JKS keystore file (Java Key Store)")
     private MultipartFile jksFile;
 
-    @Schema(description = "The password for the keystore or the private key")
+    @Schema(description = "The password for the keystore or the private key", format = "password")
     private String password;
 
     @Schema(description = "Whether to visually show the signature in the PDF file")
-    private boolean showSignature;
+    private Boolean showSignature;
 
     @Schema(description = "The reason for signing the PDF")
     private String reason;
@@ -49,9 +50,10 @@ public class SignPDFWithCertRequest extends PDFFile {
 
     @Schema(
             description =
-                    "The page number where the signature should be visible. This is required if showSignature is set to true")
+                    "The page number where the signature should be visible. This is required if"
+                            + " showSignature is set to true")
     private Integer pageNumber;
 
     @Schema(description = "Whether to visually show a signature logo along with the signature")
-    private boolean showLogo;
+    private Boolean showLogo;
 }