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/Capability.java

@@ -27,7 +27,7 @@ public final class Capability {
     /**
      * If UNSET, the server supports the MariaDB protocol and statements.
      */
-    private static final long CLIENT_MYSQL = 1L;
+    static final long CLIENT_MYSQL = 1L;
 
     /**
      * Use found/touched rows instead of changed rows for affected rows. Should enable it by default.

src/main/java/io/asyncer/r2dbc/mysql/ConnectionContext.java

@@ -37,6 +37,8 @@ public final class ConnectionContext implements CodecContext {
 
     private static final ServerVersion NONE_VERSION = ServerVersion.create(0, 0, 0);
 
+    private static final Capability NONE_CAPABILITY = Capability.of(Capability.CLIENT_MYSQL);
+
     private volatile int connectionId = -1;
 
     private volatile ServerVersion serverVersion = NONE_VERSION;
@@ -57,7 +59,7 @@ public final class ConnectionContext implements CodecContext {
      */
     private volatile short serverStatuses = ServerStatuses.AUTO_COMMIT;
 
-    private volatile Capability capability = null;
+    private volatile Capability capability = NONE_CAPABILITY;
 
     ConnectionContext(ZeroDateOption zeroDateOption, @Nullable Path localInfilePath,
         int localInfileBufferSize, @Nullable ZoneId serverZoneId) {