Support decrypting AES data where only IV is present

Author: Arnaud TAMAILLON

src/UglyToad.PdfPig.Tests/Integration/EncryptedDocumentTests.cs

@@ -53,6 +53,15 @@ public void CanReadDocumentWithUEAsString()
                 Assert.NotNull(document.Information.Producer);
             }
         }
+        
+        [Fact]
+        public void CanReadDocumentWithEmptyStringEncryptedWithAESEncryptionAndOnlyIV()
+        {
+            using (var document = PdfDocument.Open(IntegrationHelpers.GetSpecificTestDocumentPath("r4_aes_empty_string.pdf")))
+            {
+                Assert.Empty(document.Information.Producer);
+            }
+        }
 
         private static string GetPath() => IntegrationHelpers.GetSpecificTestDocumentPath(FileName);
     }

src/UglyToad.PdfPig.Tests/Integration/SpecificTestDocuments/r4_aes_empty_string.pdf

@@ -27,7 +27,12 @@ public static byte[] Decrypt(byte[] data, byte[] finalKey)
                 aes.IV = iv;
 
 #if NET8_0_OR_GREATER
-                return aes.DecryptCbc(data.AsSpan(iv.Length), iv, PaddingMode.PKCS7);
+                var encryptedData = data.AsSpan(iv.Length);
+                if (encryptedData.IsEmpty)
+                {
+                    return [];
+                }
+                return aes.DecryptCbc(encryptedData, iv, PaddingMode.PKCS7);
 #else
                 var buffer = new byte[256];