Fixed NPE with SslMode.TUNNEL Usage

Motivation:
A NPE was identified when utilizing `SslMode.TUNNEL`, introduced by PR #204. The issue arises when `ConnectionContext#isMariaDb` is invoked from `SslBridgeHandler#isTls13Enabled`, leading to an NPE due to the ConnectionContext not being initialized at that time.

Modification:
We have updated ConnectionContext#isMariaDb to return false if the context has not been initialized, preventing the NPE. (Mainly to restore previous behavior)

Result:
This change addresses the NPE issue, ensuring stability when `SslMode.TUNNEL` is selected. It resolves the problem reported in GoogleCloudPlatform/cloud-sql-jdbc-socket-factory#1828

Author: jchrys

src/main/java/io/asyncer/r2dbc/mysql/client/SslBridgeHandler.java

@@ -222,7 +222,7 @@ static MySqlSslContextSpec forClient(MySqlSslConfiguration ssl, ConnectionContex
 
             if (tlsProtocols.length > 0) {
                 builder.protocols(tlsProtocols);
-            } else if (isTls13Enabled(context)) {
+            } else if (ssl.getSslMode() == SslMode.TUNNEL|| isTls13Enabled(context)) {
                 builder.protocols(TLS_PROTOCOLS);
             } else {
                 // Not sure if we need to check the JDK version, suggest not.

src/test/java/io/asyncer/r2dbc/mysql/SslTunnelIntegrationTest.java

@@ -58,7 +58,7 @@ public class SslTunnelIntegrationTest {
     public void setUp() throws CertificateException, SSLException {
         server = new SelfSignedCertificate();
         client = new SelfSignedCertificate();
-        final SslContext sslContext = SslContextBuilder.forServer(server.key(), server.cert()).build();
+        final SslContext sslContext = SslContextBuilder.forServer(server.key(), server.cert()).protocols(TlsVersions.TLS1_1, TlsVersions.TLS1, TlsVersions.TLS1_2, TlsVersions.TLS1_3).build();
         sslTunnelServer = new SslTunnelServer("localhost", 3306, sslContext);
         sslTunnelServer.setUp();
     }
@@ -87,7 +87,6 @@ public void sslTunnelConnectionTest() {
                 .database("r2dbc")
                 .createDatabaseIfNotExist(true)
                 .sslMode(SslMode.TUNNEL)
-                .tlsVersion(TlsVersions.TLS1_3, TlsVersions.TLS1_2)
                 .sslKey(client.privateKey().getAbsolutePath())
                 .sslCert(client.certificate().getAbsolutePath())
                 .sslCa(server.certificate().getAbsolutePath())