[hotfix] Fix that VariantType doesn't have default constructor (apache#6162)

Author: yuzelin

paimon-flink/paimon-flink-2.0/src/main/java/org/apache/flink/table/types/logical/VariantType.java

@@ -36,6 +36,10 @@ public VariantType(boolean isNullable) {
         super(isNullable, LogicalTypeRoot.UNRESOLVED);
     }
 
+    public VariantType() {
+        this(true);
+    }
+
     @Override
     public LogicalType copy(boolean b) {
         return null;

paimon-flink/paimon-flink1-common/src/main/java/org/apache/flink/table/types/logical/VariantType.java

@@ -36,6 +36,10 @@ public VariantType(boolean isNullable) {
         super(isNullable, LogicalTypeRoot.UNRESOLVED);
     }
 
+    public VariantType() {
+        this(true);
+    }
+
     @Override
     public LogicalType copy(boolean b) {
         return null;