Fix InputDecorationTheme copyWith fallback for iconColor (#142462)

Same as https://github.com/flutter/flutter/pull/138914, but with a test.

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

@@ -4309,7 +4309,7 @@ class InputDecorationTheme with Diagnosticable {
       floatingLabelAlignment: floatingLabelAlignment ?? this.floatingLabelAlignment,
       isDense: isDense ?? this.isDense,
       contentPadding: contentPadding ?? this.contentPadding,
-      iconColor: iconColor,
+      iconColor: iconColor ?? this.iconColor,
       isCollapsed: isCollapsed ?? this.isCollapsed,
       prefixStyle: prefixStyle ?? this.prefixStyle,
       prefixIconColor: prefixIconColor ?? this.prefixIconColor,

packages/flutter/test/material/input_decorator_test.dart

@@ -7047,4 +7047,16 @@ testWidgets('OutlineInputBorder with BorderRadius.zero should draw a rectangular
     await tester.pumpAndSettle();
     expect(getLabelStyle(tester).height, beforeStyle.height);
   });
+
+  test('InputDecorationTheme.copyWith keeps original iconColor.', () async {
+    const InputDecorationTheme original = InputDecorationTheme(iconColor: Color(0xDEADBEEF));
+    expect(original.iconColor, const Color(0xDEADBEEF));
+    expect(original.fillColor, isNot(const Color(0xDEADCAFE)));
+    final InputDecorationTheme copy1 = original.copyWith(fillColor: const Color(0xDEADCAFE));
+    expect(copy1.iconColor, const Color(0xDEADBEEF));
+    expect(copy1.fillColor, const Color(0xDEADCAFE));
+    final InputDecorationTheme copy2 = original.copyWith(iconColor: const Color(0xDEADCAFE));
+    expect(copy2.iconColor, const Color(0xDEADCAFE));
+    expect(copy2.fillColor, isNot(const Color(0xDEADCAFE)));
+  });
 }