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/ConnectionContext.java

@@ -57,6 +57,7 @@ public final class ConnectionContext implements CodecContext {
      */
     private volatile short serverStatuses = ServerStatuses.AUTO_COMMIT;
 
+    @Nullable
     private volatile Capability capability = null;
 
     ConnectionContext(ZeroDateOption zeroDateOption, @Nullable Path localInfilePath,
@@ -109,7 +110,8 @@ public ZoneId getServerZoneId() {
 
     @Override
     public boolean isMariaDb() {
-        return capability.isMariaDb() || serverVersion.isMariaDb();
+        final Capability c = capability;
+        return c != null && (c.isMariaDb() || serverVersion.isMariaDb());
     }
 
     boolean shouldSetServerZoneId() {