Make default scale value null so that Flutter decides the one to use (#247)

Co-authored-by: Daichi Furiya <[email protected]>

Author: davidmigloz

packages/core/lib/generators/assets_generator.dart

@@ -369,7 +369,7 @@ class AssetGenImage {
     ImageErrorWidgetBuilder? errorBuilder,
     String? semanticLabel,
     bool excludeFromSemantics = false,
-    double? scale = 1.0,
+    double? scale,
     double? width,
     double? height,
     Color? color,