fix nil value conversion (open-telemetry#34673)

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

fix nil value conversion

**Link to tracking Issue:** <Issue number if applicable>

Fixes open-telemetry#34672 

**Testing:** <Describe what testing was performed and which tests were
added.>

added.

**Documentation:** <Describe the documentation added.>

n/a

.chloggen/pkg-stanza-nil-handling.yaml

@@ -0,0 +1,27 @@
+# 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. filelogreceiver)
+component: pkg/stanza
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: fix nil value conversion
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [34672]
+
+# (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:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# 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: []

pkg/stanza/adapter/converter.go

@@ -322,6 +322,7 @@ func upsertToAttributeVal(value any, dest pcommon.Value) {
 		upsertToMap(t, dest.SetEmptyMap())
 	case []any:
 		upsertToSlice(t, dest.SetEmptySlice())
+	case nil:
 	default:
 		dest.SetStr(fmt.Sprintf("%v", t))
 	}

pkg/stanza/adapter/converter_test.go

@@ -566,6 +566,7 @@ func TestConvertMetadata(t *testing.T) {
 			"int":    123,
 			"double": 12.34,
 			"string": "hello",
+			"empty":  nil,
 		},
 	}
 	e.Body = true
@@ -595,7 +596,7 @@ func TestConvertMetadata(t *testing.T) {
 	require.True(t, ok)
 
 	mapVal := attVal.Map()
-	require.Equal(t, 4, mapVal.Len())
+	require.Equal(t, 5, mapVal.Len())
 
 	attVal, ok = mapVal.Get("bool")
 	require.True(t, ok)
@@ -613,6 +614,10 @@ func TestConvertMetadata(t *testing.T) {
 	require.True(t, ok)
 	require.Equal(t, "hello", attVal.Str())
 
+	attVal, ok = mapVal.Get("empty")
+	require.True(t, ok)
+	require.Equal(t, pcommon.ValueTypeEmpty, attVal.Type())
+
 	bod := result.Body()
 	require.Equal(t, pcommon.ValueTypeBool, bod.Type())
 	require.True(t, bod.Bool())