[KYUUBI #6534] Fix checkEngineConnectionAlive may throw NPE

# 🔍 Description
## Issue References 🔗

This pull request fixes #6534

## Describe Your Solution 🔧

check null ahead.

## Types of changes 🔖

- [x] Bugfix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to change)

## Test Plan 🧪

Pass GHA

---

# Checklist 📝

- [x] This patch was not authored or co-authored using [Generative Tooling](https://www.apache.org/legal/generative-tooling.html)

**Be nice. Be informative.**

Closes #6540 from pan3793/client-npe.

Closes #6534

6976cd3 [Cheng Pan] [KYUUBI #6534] Fix checkEngineConnectionAlive may throw NPE

Authored-by: Cheng Pan <[email protected]>
Signed-off-by: Cheng Pan <[email protected]>
(cherry picked from commit 7336fbc)
Signed-off-by: Cheng Pan <[email protected]>

Author: pan3793

kyuubi-server/src/main/scala/org/apache/kyuubi/session/KyuubiSessionImpl.scala

@@ -315,7 +315,8 @@ class KyuubiSessionImpl(
   }
 
   def checkEngineConnectionAlive(): Boolean = {
-    if (Option(client).exists(_.engineConnectionClosed)) return false
+    if (client == null) return true // client has not been initialized
+    if (client.engineConnectionClosed) return false
     !client.remoteEngineBroken
   }
 }