Define handling of null and empty attribute values (#459)
Define handling of null and empty attribute values (#459)

* Define handling of null and empty attribute values

* undefined -> not set

* Apply suggestions from code review

Co-Authored-By: Wolfgang Ziegler <43dc04569bfcf1df8bbd6902f9ec662131e3271b@dynatrace.com>

* Apply suggestions from code review

Co-Authored-By: Christian Neumüller <00e647e70423c7414ea66f19db6b37f3e922a1b3@dynatrace.com>

* Allow exporters to replace null values in arrays

* Apply suggestions from code review

Co-Authored-By: Christian Neumüller <00e647e70423c7414ea66f19db6b37f3e922a1b3@dynatrace.com>

* attribute semantics -> restrictions

* emphasize MUST

Co-authored-by: Wolfgang Ziegler <6701d2676c4160a96afcd7a600ebc2d0c7be0e06@live.at>
Co-authored-by: Christian Neumüller <f80c045e0b29a8a0d8cfc19e571e9d9523c3f1c5@neumueller.me>

open-telemetry/opentelemetry-specification,open-telemetry/opentelemetry-specification