Atbash Cipher exercise needs tests for empty input

The Atbash Cipher needs additional tests for empty strings...

```
TEST_CASE("encode_empty_string")
{
REQUIRE("" == atbash::encode(""));
}
TEST_CASE("decode_empty_string")
{
REQUIRE("" == atbash::decode(""));
}

```
In particular student code can fail encode empty string with improper handling of trailing white space.