(#3765) Don't try to decrypt null strings

The DefaultEncryptionUtility attempts to decrypt a string without
checking if the string is null or empty. This commit adds a check the
same as in EncryptString whereby we return null if the input string is
null or empty.

Author: corbob

src/chocolatey/infrastructure/cryptography/DefaultEncryptionUtility.cs

@@ -61,6 +61,11 @@ This is can be because the machine keyfile cannot be written as a normal user.
 
         public string DecryptString(string encryptedString)
         {
+            if (string.IsNullOrEmpty(encryptedString))
+            {
+                return null;
+            }
+
             var encryptedByteArray = Convert.FromBase64String(encryptedString);
             byte[] decryptedByteArray;