Remove null from flex documentation (flutter#158086)

Closes flutter#158085

Author: ValentinVignal

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

@@ -5359,10 +5359,10 @@ class Flexible extends ParentDataWidget<FlexParentData> {
 
   /// The flex factor to use for this child.
   ///
-  /// If null or zero, the child is inflexible and determines its own size. If
-  /// non-zero, the amount of space the child can occupy in the main axis is
-  /// determined by dividing the free space (after placing the inflexible
-  /// children) according to the flex factors of the flexible children.
+  /// If zero, the child is inflexible and determines its own size. If non-zero,
+  /// the amount of space the child can occupy in the main axis is determined by
+  /// dividing the free space (after placing the inflexible children) according
+  /// to the flex factors of the flexible children.
   final int flex;
 
   /// How a flexible child is inscribed into the available space.