fix(pdf-download): avoid empty abstract in PDF output

Added conditional check to ensure the abstract is only included in the PDF if it exists. This prevents empty or malformed sections when post excerpts are not provided, improving the readability and quality of the generated PDF documents.

functions.php

@@ -885,10 +885,12 @@ function handle_pdf_download()
                 }
 
                 // Set abstract
+                if ($post->post_excerpt) {
                     $pdf->Ln(10);
                     $pdf->SetFont('helvetica', '', 12);
                     $pdf->setTextColor(128);
                     $pdf->writeHTML("<b>Abstract:</b> " . $post->post_excerpt);
+                }
 
                 // Set content
                 $pdf->setTextColor(0);