Prevent crash when calling lerp for IconThemeData with null arguments (#26592)

The lerp method for IconThemeData was not doing anything to prevent exceptions when null arguments are used. This change fixes that and adds a test for it.

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

@@ -72,9 +72,9 @@ class IconThemeData extends Diagnosticable {
   static IconThemeData lerp(IconThemeData a, IconThemeData b, double t) {
     assert(t != null);
     return IconThemeData(
-      color: Color.lerp(a.color, b.color, t),
-      opacity: ui.lerpDouble(a.opacity, b.opacity, t),
-      size: ui.lerpDouble(a.size, b.size, t),
+      color: Color.lerp(a?.color, b?.color, t),
+      opacity: ui.lerpDouble(a?.opacity, b?.opacity, t),
+      size: ui.lerpDouble(a?.size, b?.size, t),
     );
   }
 

packages/flutter/test/widgets/icon_theme_data_test.dart

@@ -14,8 +14,33 @@ void main() {
     expect(data.hashCode, equals(data.copyWith().hashCode));
 
     final IconThemeData lerped = IconThemeData.lerp(data, const IconThemeData.fallback(), 0.25);
-    expect(lerped.color, equals(Color.lerp(const Color(0xAAAAAAAA), const Color(0xFF000000), 0.25)));
-    expect(lerped.opacity, equals(0.625));
-    expect(lerped.size, equals(18.0));
+    expect(lerped.color, const Color(0xBF7F7F7F));
+    expect(lerped.opacity, 0.625);
+    expect(lerped.size, 18.0);
+  });
+
+  test('IconThemeData lerp with first null', () {
+    const IconThemeData data = IconThemeData(color: Color(0xFFFFFFFF), opacity: 1.0, size: 16.0);
+
+    final IconThemeData lerped = IconThemeData.lerp(null, data, 0.25);
+    expect(lerped.color, const Color(0x40FFFFFF));
+    expect(lerped.opacity, 0.25);
+    expect(lerped.size, 4.0);
+  });
+
+  test('IconThemeData lerp with second null', () {
+    const IconThemeData data = IconThemeData(color: Color(0xFFFFFFFF), opacity: 1.0, size: 16.0);
+
+    final IconThemeData lerped = IconThemeData.lerp(data, null, 0.25);
+    expect(lerped.color, const Color(0xBFFFFFFF));
+    expect(lerped.opacity, 0.75);
+    expect(lerped.size, 12.0);
+  });
+
+  test('IconThemeData lerp with both null', () {
+    final IconThemeData lerped = IconThemeData.lerp(null, null, 0.25);
+    expect(lerped.color, null);
+    expect(lerped.opacity, null);
+    expect(lerped.size, null);
   });
 }