fix: Do not crash if a certificate key ID is not known

Fixes #17.

libs/lpac-jni/src/main/java/net/typeblog/lpac_jni/impl/RootCertificates.kt

@@ -8,7 +8,7 @@ import java.security.cert.CertificateFactory
 const val DEFAULT_PKID_GSMA_RSP2_ROOT_CI1 = "81370f5125d0b1d408d4c3b232e6d25e795bebfb"
 
 private fun getCertificate(keyId: String): Certificate? =
-    KNOWN_CI_CERTS[keyId]?.toByteArray().let { cert ->
+    KNOWN_CI_CERTS[keyId]?.toByteArray()?.let { cert ->
         ByteArrayInputStream(cert).use { stream ->
             val cf = CertificateFactory.getInstance("X.509")
             cf.generateCertificate(stream)