fix null pointer exception when using public only keys (#3143)

[gregrenda]

Don't call PGPSecretKeyRingCollection for public only keys

[msfjarvis]

Please add a test for it here https://github.com/android-password-store/Android-Password-Store/blob/develop/crypto/pgpainless/src/test/kotlin/app/passwordstore/crypto/PGPainlessCryptoHandlerTest.kt

[msfjarvis]

I had some time so I tried to adapt an existing test to reproduce this but couldn't get it to fail without your fix, feel free to build upon this or write your own test. It should fail on develop and pass on this branch.

  @Ignore("Should fail when decrypting but completes successfully")
  @Test
  fun decryptWithPublicKeys() {
    val aliceSecKeyRing =
      PGPainless.generateKeyRing().modernKeyRing("Alice <owner@example.com>", KEY_PASSPHRASE)
    val bobSecKeyRing = PGPainless.generateKeyRing().modernKeyRing("Bob <owner@example.com>", KEY_PASSPHRASE)
    val aliceCert = PGPainless.extractCertificate(aliceSecKeyRing)
    val aliceKey = PGPKey(PGPainless.asciiArmor(aliceCert).encodeToByteArray())
    val bobKey = PGPKey(PGPainless.asciiArmor(bobSecKeyRing).encodeToByteArray())

    val ciphertextStream = ByteArrayOutputStream()
    val encryptRes =
      cryptoHandler.encrypt(
        listOf(aliceKey, bobKey),
        PLAIN_TEXT.byteInputStream(Charsets.UTF_8),
        ciphertextStream,
        PGPEncryptOptions.Builder().withAsciiArmor(true).build(),
      )
    assertTrue(encryptRes.isOk)

    val message = ciphertextStream.toByteArray().decodeToString()
    val info = MessageInspector.determineEncryptionInfoForMessage(message)
    assertTrue(info.isEncrypted)
    assertEquals(2, info.keyIds.size)
    assertFalse(info.isSignedOnly)

    assertFails {
      val ciphertextStreamCopy = message.byteInputStream()
      val plaintextStream = ByteArrayOutputStream()
      cryptoHandler.decrypt(
        listOf(aliceKey, bobKey),
        KEY_PASSPHRASE,
        ciphertextStreamCopy,
        plaintextStream,
        PGPDecryptOptions.Builder().build(),
      )
    }
  }

[gregrenda]

Added a unit test. You have to pass both private and public keys to decrypt simultaneously to trigger the bug. You probably shouldn't be passing public keys to decrypt but that's likely a bigger change.