Make Decoration.padding non-nullable (#119581)

The default implementation returns EdgeInsets.zero, the ShapeDecoration subclass already makes it non-nullable, and there isn't any benefit to returning null as far as I can tell.

packages/flutter/lib/src/material/ink_decoration.dart

@@ -237,10 +237,10 @@ class Ink extends StatefulWidget {
   final double? height;
 
   EdgeInsetsGeometry get _paddingIncludingDecoration {
-    if (decoration == null || decoration!.padding == null) {
+    if (decoration == null) {
       return padding ?? EdgeInsets.zero;
     }
-    final EdgeInsetsGeometry decorationPadding = decoration!.padding!;
+    final EdgeInsetsGeometry decorationPadding = decoration!.padding;
     if (padding == null) {
       return decorationPadding;
     }

packages/flutter/lib/src/painting/box_decoration.dart

@@ -208,7 +208,7 @@ class BoxDecoration extends Decoration {
   final BoxShape shape;
 
   @override
-  EdgeInsetsGeometry? get padding => border?.dimensions;
+  EdgeInsetsGeometry get padding => border?.dimensions ?? EdgeInsets.zero;
 
   @override
   Path getClipPath(Rect rect, TextDirection textDirection) {

packages/flutter/lib/src/painting/decoration.dart

@@ -59,7 +59,7 @@ abstract class Decoration with Diagnosticable {
   /// [EdgeInsetsGeometry.resolve] to obtain an absolute [EdgeInsets]. (For
   /// example, [BorderDirectional] will return an [EdgeInsetsDirectional] for
   /// its [padding].)
-  EdgeInsetsGeometry? get padding => EdgeInsets.zero;
+  EdgeInsetsGeometry get padding => EdgeInsets.zero;
 
   /// Whether this decoration is complex enough to benefit from caching its painting.
   bool get isComplex => false;

packages/flutter/lib/src/widgets/container.dart

@@ -367,14 +367,14 @@ class Container extends StatelessWidget {
   final Clip clipBehavior;
 
   EdgeInsetsGeometry? get _paddingIncludingDecoration {
-    if (decoration == null || decoration!.padding == null) {
+    if (decoration == null) {
       return padding;
     }
-    final EdgeInsetsGeometry? decorationPadding = decoration!.padding;
+    final EdgeInsetsGeometry decorationPadding = decoration!.padding;
     if (padding == null) {
       return decorationPadding;
     }
-    return padding!.add(decorationPadding!);
+    return padding!.add(decorationPadding);
   }
 
   @override

packages/flutter/test/widgets/animated_container_test.dart

@@ -80,10 +80,16 @@ void main() {
         ' │   PlatformAssetBundle#00000(), devicePixelRatio: 3.0, platform:\n'
         ' │   android)\n'
         ' │\n'
-        ' └─child: RenderLimitedBox#00000\n'
+        ' └─child: RenderPadding#00000\n'
         '   │ parentData: <none> (can use size)\n'
         '   │ constraints: BoxConstraints(w=800.0, h=600.0)\n'
         '   │ size: Size(800.0, 600.0)\n'
+        '   │ padding: EdgeInsets.zero\n'
+        '   │\n'
+        '   └─child: RenderLimitedBox#00000\n'
+        '     │ parentData: offset=Offset(0.0, 0.0) (can use size)\n'
+        '     │ constraints: BoxConstraints(w=800.0, h=600.0)\n'
+        '     │ size: Size(800.0, 600.0)\n'
         '     │ maxWidth: 0.0\n'
         '     │ maxHeight: 0.0\n'
         '     │\n'