instrumentation config: return empty object when not set (#4817)

Fixes open-telemetry/opentelemetry-java#7958

## Changes

While working on Java, it has turned out to be more convenient to work
with empty config objects rather than null/nil.

- null requires an explicit if, making the code more verbose
- it doesn't matter if the `.instrumentation` is missing or is empty,
which is a different question than "is declarative config used")

* [x] Related issues:
open-telemetry/opentelemetry-java#7927 (comment)
* [ ] Related [OTEP(s)](https://github.com/open-telemetry/oteps) #
* [x] Links to the prototypes (when adding or changing features):
open-telemetry/opentelemetry-java#7954
* [ ]
[`CHANGELOG.md`](https://github.com/open-telemetry/opentelemetry-specification/blob/main/CHANGELOG.md)
file updated for non-trivial changes
* [ ] [Spec compliance
matrix](https://github.com/open-telemetry/opentelemetry-specification/blob/main/spec-compliance-matrix/template.yaml)
updated if necessary

---------

Co-authored-by: Trask Stalnaker <[email protected]>
Co-authored-by: Jack Berg <[email protected]>
Co-authored-by: Carlos Alberto Cortez <[email protected]>

Author: 4 people

CHANGELOG.md

@@ -69,6 +69,9 @@ release.
   ([#4802](https://github.com/open-telemetry/opentelemetry-specification/pull/4802))
 - Declarative configuration: clarify Registry ComponentProvider `type` parameter
   ([#4799](https://github.com/open-telemetry/opentelemetry-specification/pull/4799))
+- Declarative configuration: Update instrumentation config behavior to return
+  empty object when not set
+  ([#4817](https://github.com/open-telemetry/opentelemetry-specification/pull/4817))
 - Swap Tracer/Meter/LoggerConfig `disabled` for `enabled` to avoid double negatives
   ([#4823](https://github.com/open-telemetry/opentelemetry-specification/pull/4823))
 

specification/configuration/api.md

@@ -60,9 +60,8 @@ Obtain configuration relevant to instrumentation libraries.
 the [`.instrumentation`](https://github.com/open-telemetry/opentelemetry-configuration/blob/670901762dd5cce1eecee423b8660e69f71ef4be/examples/kitchen-sink.yaml#L438-L439)
 configuration mapping node.
 
-If the `.instrumentation` node is not set, get instrumentation config MUST
-return nil, null, undefined or another language-specific idiomatic pattern
-denoting empty.
+If the `.instrumentation` node is not set, get instrumentation config SHOULD
+return an empty `ConfigProperties` (as if `.instrumentation: {}` was set).
 
 ### ConfigProperties