Use boost::empty_value from Core for EBO

[glenfe]

No description provided.

[codecov]

Codecov Report

Merging #1233 into develop will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #1233      +/-   ##
===========================================
- Coverage    95.52%   95.51%   -0.01%     
===========================================
  Files          113      112       -1     
  Lines        11064    11042      -22     
===========================================
- Hits         10569    10547      -22     
  Misses         495      495

Impacted Files	Coverage Δ
include/boost/beast/http/impl/message.ipp	96.03% <ø> (ø)	⬆️
include/boost/beast/http/fields.hpp	92.3% <100%> (ø)	⬆️
include/boost/beast/http/message.hpp	100% <100%> (ø)	⬆️
...lude/boost/beast/http/detail/basic_parsed_list.hpp	100% <100%> (ø)	⬆️
include/boost/beast/core/impl/flat_buffer.ipp	99.5% <100%> (ø)	⬆️
include/boost/beast/http/impl/fields.ipp	98.13% <100%> (ø)	⬆️
include/boost/beast/core/impl/multi_buffer.ipp	98.61% <100%> (ø)	⬆️
include/boost/beast/core/multi_buffer.hpp	100% <100%> (ø)	⬆️
include/boost/beast/core/flat_buffer.hpp	100% <100%> (ø)	⬆️

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