Define "none" enum value (open-telemetry#2102)

Co-authored-by: Joshua MacDonald <[email protected]>

specification/sdk-environment-variables.md

@@ -23,6 +23,8 @@ If an SDK chooses to support an integer-valued environment variable, it SHOULD s
 For variables which accept a known value out of a set, i.e., an enum value, SDK implementations MAY support additional values not listed here.
 For variables accepting an enum value, if the user provides a value the SDK does not recognize, the SDK MUST generate a warning and gracefully ignore the setting.
 
+If a null object (empty, no-op) value is acceptable, then the enum value representing it MUST be `"none"`.
+
 ### Duration
 
 Any value that represents a duration, for example a timeout, MUST be an integer representing a number of