fix: Root spans must have a non-empty parent ID field (#1236)

## Which problem is this PR solving?

Span's with a `parentID` field with an empty value `""` are considered
root spans. This PR updates the logic to determine root spans to check
that both the parentID field exists and is not empty for it to be
considered a root span.

- Fixes #1234 

## Short description of the changes
- Check the parentID field is both present and has a value when checking
if the span is a root span
- Update tests to verify updated behaviour

Co-authored-by: Kent Quirk <kentquirk@honeycomb.io>

collect/collect.go

@@ -601,7 +601,7 @@ func (i *InMemCollector) isRootSpan(sp *types.Span) bool {
 	// check if the event has a parent id using the configured parent id field names
 	for _, parentIdFieldName := range i.Config.GetParentIdFieldNames() {
 		parentId := sp.Data[parentIdFieldName]
-		if _, ok := parentId.(string); ok {
+		if _, ok := parentId.(string); ok && parentId != "" {
 			return false
 		}
 	}

collect/collect_test.go

@@ -1384,15 +1384,15 @@ func TestIsRootSpan(t *testing.T) {
 			expected: true,
 		},
 		{
-			name: "non-root span - empty parent id",
+			name: "root span - empty parent id",
 			span: &types.Span{
 				Event: types.Event{
 					Data: map[string]interface{}{
 						"trace.parent_id": "",
 					},
 				},
 			},
-			expected: false,
+			expected: true,
 		},
 		{
 			name: "non-root span - parent id",