fix(Cert): ensure incremental save after PDF signing in CertSignController (#5390)

# Description of Changes

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## Checklist

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- [ ] I have read the [Contribution
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(if applicable)
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- [ ] My changes generate no new warnings

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app/core/src/main/java/stirling/software/SPDF/controller/api/security/CertSignController.java

@@ -139,11 +139,12 @@ public class CertSignController {
                     signatureOptions.setPage(pageNumber);
 
                     doc.addSignature(signature, instance, signatureOptions);
+                    doc.saveIncremental(output);
                 }
             } else {
                 doc.addSignature(signature, instance);
+                doc.saveIncremental(output);
             }
-            doc.saveIncremental(output);
         } catch (Exception e) {
             ExceptionUtils.logException("PDF signing", e);
         }