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

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

@@ -220,11 +220,13 @@ static MySqlSslContextSpec forClient(MySqlSslConfiguration ssl, ConnectionContex
                 .applicationProtocolConfig(null);
             String[] tlsProtocols = ssl.getTlsVersion();
 
-            if (tlsProtocols.length > 0) {
-                builder.protocols(tlsProtocols);
-            } else if (ssl.getSslMode() != SslMode.TUNNEL && isTls13Enabled(context)) {
+            if (tlsProtocols.length > 0 || ssl.getSslMode() == SslMode.TUNNEL) {
+                if (tlsProtocols.length > 0) {
+                    builder.protocols(tlsProtocols);
+                }
+            } else if (isTls13Enabled(context)) {
                 builder.protocols(TLS_PROTOCOLS);
-            } else if (ssl.getSslMode() != SslMode.TUNNEL){
+            } else {
                 // Not sure if we need to check the JDK version, suggest not.
                 if (logger.isWarnEnabled()) {
                     logger.warn("{} {} does not support TLS1.2, TLS1.1 is disabled in latest JDKs",