Add facade lifecycle wrappers

internal/facade/diagnostic.go

@@ -20,6 +20,9 @@ func (e *LoadError) Diagnostics() []Diagnostic {
 		return nil
 	}
 	values := e.value.Diagnostics()
+	if values == nil {
+		return nil
+	}
 	result := make([]Diagnostic, len(values))
 	for i := range values {
 		result[i] = fromEngineDiagnostic(values[i])
@@ -31,6 +34,9 @@ func (e *LoadError) Sources() []SourceInfo {
 		return nil
 	}
 	values := e.value.Sources()
+	if values == nil {
+		return nil
+	}
 	result := make([]SourceInfo, len(values))
 	for i := range values {
 		result[i] = fromEngineSourceInfo(values[i])