[Bugfix] be crash when executing sql contains bitmap_intersect function (#9910)

* fix bitmap serialize bug

* add regression test for bitmap seralize bugfix

* add missing regression test out file

* fix reggresion test failed issue

be/src/exprs/bitmap_function.cpp

@@ -393,7 +393,8 @@ StringVal BitmapFunctions::bitmap_hash(doris_udf::FunctionContext* ctx,
 
 StringVal BitmapFunctions::bitmap_serialize(FunctionContext* ctx, const StringVal& src) {
     if (src.is_null) {
-        return src;
+        // bitmap functions should never return nullable value
+        return serialize(ctx, nullptr);
     }
 
     auto src_bitmap = reinterpret_cast<BitmapValue*>(src.ptr);