ecies: fix tests for ecies-iso (allow empty ciphertext)
ecies: fix tests for ecies-iso (allow empty ciphertext)

Rohde-Schwarz-Cybersecurity/botan,Rohde-Schwarz-Cybersecurity/botan,webmaster128/botan,webmaster128/botan,Rohde-Schwarz-Cybersecurity/botan,Rohde-Schwarz-Cybersecurity/botan,webmaster128/botan,Rohde-Schwarz-Cybersecurity/botan,randombit/botan,Rohde-Schwarz-Cybersecurity/botan,randombit/botan,webmaster128/botan,webmaster128/botan,randombit/botan,randombit/botan,randombit/botan