[confmap] Enable DecodeNil option (#12996)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

<!-- Issue number if applicable -->

Enables `DecoderConfig.DecodeNil` 


#### Link to tracking issue

Taken from #10260
Updates #12981

Author: mx-psi

.chloggen/mx-psi_decodenil.yaml

@@ -0,0 +1,25 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: confmap
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Support running Unmarshal hooks on nil values.
+
+# One or more tracking issues or pull requests related to the change
+issues: [12981]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [api]

confmap/confmap.go

@@ -235,6 +235,7 @@ func decodeConfig(m *Conf, result any, errorUnused bool, skipTopLevelUnmarshaler
 		TagName:          MapstructureTag,
 		WeaklyTypedInput: false,
 		MatchName:        caseSensitiveMatchName,
+		DecodeNil:        true,
 		DecodeHook: composehook.ComposeDecodeHookFunc(
 			useExpandValue(),
 			expandNilStructPointersHookFunc(),
@@ -582,10 +583,7 @@ type Marshaler interface {
 func zeroSliceHookFunc() mapstructure.DecodeHookFuncValue {
 	return safeWrapDecodeHookFunc(func(from reflect.Value, to reflect.Value) (any, error) {
 		if to.CanSet() && to.Kind() == reflect.Slice && from.Kind() == reflect.Slice {
-			if from.IsNil() {
-				// input slice is nil, set output slice to nil.
-				to.Set(reflect.Zero(to.Type()))
-			} else {
+			if !from.IsNil() {
 				// input slice is not nil, set the output slice to a new slice of the same type.
 				to.Set(reflect.MakeSlice(to.Type(), from.Len(), from.Cap()))
 			}