Ensure that Nostr::Crypto#descrypt_text always returns a string

Fixes another Steep/RBS violation

lib/nostr/crypto.rb

@@ -62,6 +62,10 @@ module Nostr
     #
     def decrypt_text(recipient_private_key, sender_public_key, encrypted_text)
       base64_encoded_text, iv = encrypted_text.split('?iv=')
+
+      # Ensure iv and base64_encoded_text are not nil
+      return '' unless iv && base64_encoded_text
+
       cipher = OpenSSL::Cipher.new(CIPHER_ALGORITHM).decrypt
       cipher.iv = Base64.decode64(iv)
       cipher.key = compute_shared_key(recipient_private_key, sender_public_key)

spec/nostr/crypto_spec.rb

@@ -69,6 +69,7 @@ RSpec.describe Nostr::Crypto do
       )
     end
 
+    context 'when the encrypted text includes an iv query string' do
       it 'decrypts an encrypted text' do
         encrypted_text = crypto.encrypt_text(sender_keypair.private_key, recipient_keypair.public_key, 'Twitter Files')
         decrypted_text = crypto.decrypt_text(recipient_keypair.private_key, sender_keypair.public_key, encrypted_text)
@@ -76,4 +77,14 @@ RSpec.describe Nostr::Crypto do
         expect(decrypted_text).to eq('Twitter Files')
       end
     end
+
+    context 'when the encrypted text does not include an iv query string' do
+      it 'returns an empty string' do
+        encrypted_text = 'wrYQaHDfpOEvyJELSCg1vzsywmlJTz8NqH03eFW44s8iQs869jtSb26Lr4s23gmY?it=v38vAJ3LlJAGZxbmWU4qAg=='
+        decrypted_text = crypto.decrypt_text(recipient_keypair.private_key, sender_keypair.public_key, encrypted_text)
+
+        expect(decrypted_text).to eq('')
+      end
+    end
+  end
 end