Don't pass "null" to debugPrint. (#11265)

debugDumpLayerTree in particular was passing null in profile mode since debugLayer isn't available in profile mode.

packages/flutter/lib/src/rendering/binding.dart

@@ -309,12 +309,12 @@ abstract class RendererBinding extends BindingBase with SchedulerBinding, Servic
 
 /// Prints a textual representation of the entire render tree.
 void debugDumpRenderTree() {
-  debugPrint(RendererBinding.instance?.renderView?.toStringDeep());
+  debugPrint(RendererBinding.instance?.renderView?.toStringDeep() ?? 'Render tree unavailable.');
 }
 
 /// Prints a textual representation of the entire layer tree.
 void debugDumpLayerTree() {
-  debugPrint(RendererBinding.instance?.renderView?.debugLayer?.toStringDeep());
+  debugPrint(RendererBinding.instance?.renderView?.debugLayer?.toStringDeep() ?? 'Layer tree unavailable.');
 }
 
 /// Prints a textual representation of the entire semantics tree.