[chore] Add tests loading nil to any (open-telemetry#12998)

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

While working on open-telemetry#12981, I would have found it useful to have these
tests, since they surfaced a bug when enabling `DecodeNil`.

Author: mx-psi

cmd/mdatagen/internal/loader_test.go

@@ -371,6 +371,22 @@ func TestLoadMetadata(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "testdata/empty_test_config.yaml",
+			want: Metadata{
+				Type:                 "test",
+				GeneratedPackageName: "metadata",
+				ScopeName:            "go.opentelemetry.io/collector/cmd/mdatagen/internal",
+				ShortFolderName:      "testdata",
+				Tests:                Tests{Host: "componenttest.NewNopHost()"},
+				Status: &Status{
+					Class: "receiver",
+					Stability: map[component.StabilityLevel][]string{
+						component.StabilityLevelBeta: {"logs"},
+					},
+				},
+			},
+		},
 		{
 			name:    "testdata/invalid_type_rattr.yaml",
 			want:    Metadata{},

cmd/mdatagen/internal/testdata/empty_test_config.yaml

@@ -0,0 +1,9 @@
+type: test
+
+status:
+  class: receiver
+  stability:
+    beta: [logs]
+
+tests:
+  config:

confmap/confmap_test.go

@@ -99,6 +99,20 @@ func TestToStringMap(t *testing.T) {
 	}
 }
 
+type testConfigAny struct {
+	AnyField any `mapstructure:"any_field"`
+}
+
+func TestNilToAnyField(t *testing.T) {
+	stringMap := map[string]any{
+		"any_field": nil,
+	}
+	conf := NewFromStringMap(stringMap)
+	cfg := &testConfigAny{}
+	require.NoError(t, conf.Unmarshal(cfg))
+	assert.Nil(t, cfg.AnyField)
+}
+
 func TestExpandNilStructPointersHookFunc(t *testing.T) {
 	stringMap := map[string]any{
 		"boolean": nil,