fix return the wrong value

Pilz.Cryptography/SimpleStringCrypter.cs

@@ -35,10 +35,13 @@ namespace Pilz.Cryptography
             byte[] bytes = TextEncoding.GetBytes(key);
             byte[] array = sha1CryptoServiceProvider.ComputeHash(bytes);
 
-            var output = new byte[checked(length - 1 + 1)];
-            array.CopyTo(output, 0);
+            var output = new byte[length];
+            var lowerLength = Math.Min(array.Length, output.Length);
+            
+            for (int i = 0; i < lowerLength; i++)
+                output[i] = array[i];
 
-            return array;
+            return output;
         }
 
         private string EncryptData(string plaintext)
@@ -63,7 +66,7 @@ namespace Pilz.Cryptography
 
         public string Encrypt(string plainValue)
         {
-            return EncryptData(plainValue);
+            return EncryptData(plainValue ?? string.Empty);
         }
 
         public string Decrypt(string encryptedValue)