[Fix](load) Restrict the import of VARCHAR(0) data to avoid coredump

be/src/vec/sink/vtablet_block_convertor.cpp

@@ -211,7 +211,7 @@ Status OlapTableBlockConvertor::_internal_validate_column(
  auto string_column_checker = [&](const ColumnString* column_string) {
  size_t limit = config::string_type_length_soft_limit_bytes;
  // when type.len is negative, std::min will return overflow value, so we need to check it
- if (type.len > 0) {
+ if (type.len >= 0) {
  limit = std::min(config::string_type_length_soft_limit_bytes, type.len);
  }