Treat nil diagnostic writers as no-output entries

execute.go

@@ -111,11 +111,12 @@ func (a *App) present(outcome Outcome, path []string, streams Streams) Outcome {
 	if len(diagnostics) == 0 {
 		return outcome
 	}
+	delivered := false
 	for _, diagnostic := range diagnostics {
 		if diagnostic.Write == nil {
-			outcome = deliveryFailure(outcome, errors.New("diagnostic has no writer"))
-			return outcome
+			continue
 		}
+		delivered = true
 		writer := &checkedWriter{writer: streams.Err}
 		writeErr := diagnostic.Write(writer)
 		if writer.failure != nil {
@@ -129,7 +130,9 @@ func (a *App) present(outcome Outcome, path []string, streams Streams) Outcome {
 			return deliveryFailure(outcome, writeErr)
 		}
 	}
-	outcome.DiagnosticDelivery = DeliveryComplete
+	if delivered {
+		outcome.DiagnosticDelivery = DeliveryComplete
+	}
 	return outcome
 }
 
@@ -148,7 +151,7 @@ func (a *App) usageLine(path []string) string {
 
 func hasErrorDiagnostic(diagnostics []Diagnostic) bool {
 	for _, diagnostic := range diagnostics {
-		if diagnostic.Severity == configkit.SeverityError {
+		if diagnostic.Write != nil && diagnostic.Severity == configkit.SeverityError {
 			return true
 		}
 	}