Do not format null plan fragment

PlanFragment in StageInfo is Nullable and so it could be null. In such
case calling formatFragment method for it does not make much sense as it
simply throws NPE.

Author: kokosing

core/trino-main/src/main/java/io/trino/sql/planner/planprinter/PlanPrinter.java

@@ -450,6 +450,9 @@ public static String textDistributedPlan(
                 queryStats.getExecutionTime().convertToMostSuccinctTimeUnit()));
 
         for (StageInfo stageInfo : allStages) {
+            if (stageInfo.getPlan() == null) {
+                continue;
+            }
             builder.append(formatFragment(
                     tableScanNode -> tableInfos.get(tableScanNode.getId()),
                     dynamicFilterDomainStats,