Add more asserts to check matrix validity (flutter#31701)

## Description

These will help identify where the matrix starts to get wrong. 

Also fixed `RenderFittexBox` to no longer paint with empty child which previously triggered invalid matrix computations (NaN with dividing by 0). See also flutter#7489

## Related Issues

flutter#31650
flutter#31700
flutter#7431

## Tests

* RenderFittedBox does not paint with empty sizes

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

@@ -1228,7 +1228,8 @@ class TransformLayer extends OffsetLayer {
   /// The [transform] and [offset] properties must be non-null before the
   /// compositing phase of the pipeline.
   TransformLayer({ Matrix4 transform, Offset offset = Offset.zero })
-    : _transform = transform,
+    : assert(transform.storage.every((double value) => value.isFinite)),
+      _transform = transform,
       super(offset: offset);
 
   /// The matrix to apply.

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

@@ -2280,9 +2280,11 @@ class RenderFittedBox extends RenderProxyBox {
       final Rect sourceRect = _resolvedAlignment.inscribe(sizes.source, Offset.zero & childSize);
       final Rect destinationRect = _resolvedAlignment.inscribe(sizes.destination, Offset.zero & size);
       _hasVisualOverflow = sourceRect.width < childSize.width || sourceRect.height < childSize.height;
+      assert(scaleX.isFinite && scaleY.isFinite);
       _transform = Matrix4.translationValues(destinationRect.left, destinationRect.top, 0.0)
         ..scale(scaleX, scaleY, 1.0)
         ..translate(-sourceRect.left, -sourceRect.top);
+      assert(_transform.storage.every((double value) => value.isFinite));
     }
   }
 
@@ -2296,7 +2298,7 @@ class RenderFittedBox extends RenderProxyBox {
 
   @override
   void paint(PaintingContext context, Offset offset) {
-    if (size.isEmpty)
+    if (size.isEmpty || child.size.isEmpty)
       return;
     _updatePaintData();
     if (child != null) {

packages/flutter/test/rendering/proxy_box_test.dart

@@ -15,25 +15,32 @@ import '../flutter_test_alternative.dart';
 import 'rendering_tester.dart';
 
 void main() {
-  test('RenderFittedBox paint', () {
+  test('RenderFittedBox does not paint with empty sizes', () {
     bool painted;
-    RenderFittedBox makeFittedBox() {
+    RenderFittedBox makeFittedBox(Size size) {
       return RenderFittedBox(
         child: RenderCustomPaint(
+          preferredSize: size,
           painter: TestCallbackPainter(onPaint: () {
             painted = true;
           }),
         ),
       );
     }
 
+    // The RenderFittedBox paints if both its size and its child's size are nonempty.
     painted = false;
-    layout(makeFittedBox(), phase: EnginePhase.paint);
+    layout(makeFittedBox(Size(1, 1)), phase: EnginePhase.paint);
     expect(painted, equals(true));
 
+    // The RenderFittedBox should not paint if its child is empty-sized.
+    painted = false;
+    layout(makeFittedBox(Size.zero), phase: EnginePhase.paint);
+    expect(painted, equals(false));
+
     // The RenderFittedBox should not paint if it is empty.
     painted = false;
-    layout(makeFittedBox(), constraints: BoxConstraints.tight(Size.zero), phase: EnginePhase.paint);
+    layout(makeFittedBox(Size(1, 1)), constraints: BoxConstraints.tight(Size.zero), phase: EnginePhase.paint);
     expect(painted, equals(false));
   });