Move need_empty_fragments inside the record layer

Move need_empty_fragments inside the record layer

This flag can now be managed entirely by the new record layer code so we
move it into ossl_record_layer_st.

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19198)