[SPARK-43125][CONNECT] Fix Connect Server Can't Handle Exception With…

… Null Message

### What changes were proposed in this pull request?
Fix the bug when Connect Server throw Exception without message.

### Why are the changes needed?
Fix bug

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
Unnecessary

Closes #40780 from Hisoka-X/SPARK-43125_Exception_NPE.

Authored-by: Hisoka <fanjiaeminem@qq.com>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

connector/connect/server/src/main/scala/org/apache/spark/sql/connect/service/SparkConnectService.scala

@@ -74,6 +74,7 @@ class SparkConnectService(debug: Boolean)
   }
 
   private def buildStatusFromThrowable(st: Throwable): RPCStatus = {
+    val message = StringUtils.abbreviate(st.getMessage, 2048)
     RPCStatus
       .newBuilder()
       .setCode(RPCCode.INTERNAL_VALUE)
@@ -85,7 +86,7 @@ class SparkConnectService(debug: Boolean)
             .setDomain("org.apache.spark")
             .putMetadata("classes", compact(render(allClasses(st.getClass).map(_.getName))))
             .build()))
-      .setMessage(StringUtils.abbreviate(st.getMessage, 2048))
+      .setMessage(if (message != null) message else "")
       .build()
   }