[fix](variant) fix variant cast (#39426)

## Proposed changes
backport: https://github.com/apache/doris/pull/39377
Issue Number: close #xxx

<!--Describe your changes.-->

fe/fe-core/src/main/java/org/apache/doris/nereids/trees/expressions/Cast.java

@@ -81,6 +81,8 @@ public class Cast extends Expression implements UnaryExpression {
             return true;
         } else if (childDataType.isJsonType() || targetType.isJsonType()) {
             return true;
+        } else if (childDataType.isVariantType() || targetType.isVariantType()) {
+            return true;
         } else {
             return child().nullable();
         }