[BUG] `CompactProtocolFieldWriter` does not write empty value string in key-value pair

During debugging a Parquet issue, I found that the Parquet metadata is not correctly written by cudf. In particular, in a round trip test, a pair of metadata `{"key_str", ""}` is written then read as `{"key_str", null}` in Spark. This is due to the `CompactProtocolFieldWriter` does not write empty value string:
```
size_t CompactProtocolWriter::write(KeyValue const& k)
{
CompactProtocolFieldWriter c(*this);
c.field_string(1, k.key);
if (not k.value.empty()) { c.field_string(2, k.value); } <================ here
return c.value();
}
```

We should write all value strings even for empty value.

However, I'm not sure if this is the expected behavior of compact protocol?