[KYUUBI #6291] Avoid NPE in MySQLErrPacket

# 🔍 Description
as title
## Issue References 🔗

This pull request fixes #6291

## Describe Your Solution 🔧

Add a rule to check if it's null

## 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 🧪

#### Behavior Without This Pull Request ⚰️

#### Behavior With This Pull Request 🎉

#### Related Unit Tests

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# 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 #6292 from zhaohehuhu/dev-0411.

Closes #6291

a58eed6 [hezhao2] refactor
40ad5e0 [hezhao2] Avoid NPE in MySQLErrPacket
e02be17 [dupeng] [KYUUBI #6288] [KYUUBI  #6283] Improve the message of distribution built without enabling web ui

Lead-authored-by: hezhao2 <[email protected]>
Co-authored-by: dupeng <[email protected]>
Signed-off-by: Cheng Pan <[email protected]>
(cherry picked from commit f43cc59)
Signed-off-by: Cheng Pan <[email protected]>

kyuubi-server/src/main/scala/org/apache/kyuubi/server/mysql/MySQLGenericPackets.scala

@@ -54,7 +54,8 @@ object MySQLErrPacket {
       case kse: KyuubiSQLException if kse.getCause != null =>
         // prefer brief nested error message instead of whole stacktrace
         apply(kse.getCause)
-      case e: Exception if e.getMessage contains "NoSuchDatabaseException" =>
+      case e: Exception
+          if e.getMessage != null && e.getMessage.contains("NoSuchDatabaseException") =>
         MySQLErrPacket(1, MySQLErrorCode.ER_BAD_DB_ERROR, cause.getMessage)
       case se: SQLException if se.getSQLState == null =>
         MySQLErrPacket(1, MySQLErrorCode.ER_INTERNAL_ERROR, cause.getMessage)