fix(JSignPdfHandler): guard mergeBackgroundWithSignature against empty image path

When background is present but the signer submitted no image
(clickToSign), passing an empty string to mergeBackgroundWithSignature
caused new Imagick('') to throw. Guard the call: only merge when both
paths are non-empty; fall back to background-only or signature-only
as appropriate. Also skip --img-path entirely when signatureImagePath
is empty in GRAPHIC_AND_DESCRIPTION mode.

Signed-off-by: Vitor Mattos <1079143+vitormattos@users.noreply.github.com>

Author: vitormattos

lib/Handler/SignEngine/JSignPdfHandler.php

@@ -366,20 +366,24 @@ private function signUsingVisibleElements(string $normalizedPdf, string $hashAlg
 						$params['--bg-path'] = $signatureImagePath;
 					}
 				} elseif ($params['--l2-text'] === '""') {
-					if ($backgroundPathForElement) {
+					if ($backgroundPathForElement && $signatureImagePath) {
 						$params['--bg-path'] = $this->mergeBackgroundWithSignature(
 							$backgroundPathForElement,
 							$signatureImagePath,
 							$this->normalizeScaleFactor($scaleFactor),
 						);
-					} else {
+					} elseif ($backgroundPathForElement) {
+						$params['--bg-path'] = $backgroundPathForElement;
+					} elseif ($signatureImagePath) {
 						$params['--bg-path'] = $signatureImagePath;
 					}
 				} else {
 					if ($renderMode === SignerElementsService::RENDER_MODE_GRAPHIC_AND_DESCRIPTION) {
 						$params['--render-mode'] = SignerElementsService::RENDER_MODE_GRAPHIC_AND_DESCRIPTION;
 						$params['--bg-path'] = $backgroundPathForElement;
-						$params['--img-path'] = $signatureImagePath;
+						if ($signatureImagePath) {
+							$params['--img-path'] = $signatureImagePath;
+						}
 					} elseif ($renderMode === SignerElementsService::RENDER_MODE_SIGNAME_AND_DESCRIPTION) {
 						$params['--render-mode'] = SignerElementsService::RENDER_MODE_GRAPHIC_AND_DESCRIPTION;
 						$params['--bg-path'] = $backgroundPathForElement;