[confmap][fix] Correctly differentiate between nil and empty slices in ToStringMap

.chloggen/nil-empty-slice.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: 'bug_fix'
+
+# 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: "Correctly differentiate between a nil and empty slice in `ToStringMap`"
+
+# One or more tracking issues or pull requests related to the change
+issues: [11749]
+
+# (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

@@ -133,6 +133,10 @@ func sanitizeExpanded(a any, useOriginal bool) any {
 		return c
 	case []any:
 		var newSlice []any
+		if m == nil {
+			return newSlice
+		}
+		newSlice = []any{}
 		for _, e := range m {
 			newSlice = append(newSlice, sanitizeExpanded(e, useOriginal))
 		}

confmap/confmaptest/configtest_test.go

@@ -30,11 +30,16 @@ func TestLoadConf(t *testing.T) {
 	assert.Equal(t, map[string]any{"floating": 3.14}, cfg.ToStringMap())
 }
 
+func TestToStringMapSanitizeNil(t *testing.T) {
+	cfg, err := LoadConf(filepath.Join("testdata", "nil.yaml"))
+	require.NoError(t, err)
+	assert.Equal(t, map[string]any{"slice": nil}, cfg.ToStringMap())
+}
+
 func TestToStringMapSanitizeEmptySlice(t *testing.T) {
 	cfg, err := LoadConf(filepath.Join("testdata", "empty-slice.yaml"))
 	require.NoError(t, err)
-	var nilSlice []interface{}
-	assert.Equal(t, map[string]any{"slice": nilSlice}, cfg.ToStringMap())
+	assert.Equal(t, map[string]any{"slice": []interface{}{}}, cfg.ToStringMap())
 }
 
 func TestValidateProviderScheme(t *testing.T) {

confmap/confmaptest/testdata/empty-slice.yaml

@@ -1 +1 @@
-slice: [] # empty slices are sanitized to nil in ToStringMap
+slice: []

confmap/confmaptest/testdata/nil.yaml

@@ -0,0 +1 @@
+slice: