Fix doc comment about onPressed for various buttons (#7555)

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

@@ -21,8 +21,8 @@ import 'theme.dart';
 /// corners. Avoid using flat buttons where they would blend in with other
 /// content, for example in the middle of lists.
 ///
-/// If the [onPressed] callback is not specified or null, then the button will
-/// be disabled, will not react to touch, and will be colored as specified by
+/// If the [onPressed] callback is null, then the button will be disabled,
+/// will not react to touch, and will be colored as specified by
 /// the [disabledColor] property instead of the [color] property. If you are
 /// trying to change the button's [color] and it is not having any effect, check
 /// that you are passing a non-null [onPressed] handler.

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

@@ -29,8 +29,8 @@ final Object _kDefaultHeroTag = new Object();
 /// buttons should be used for positive actions such as "create", "share", or
 /// "navigate".
 ///
-/// If the [onPressed] callback is not specified or null, then the button will
-/// be disabled and will not react to touch.
+/// If the [onPressed] callback is null, then the button will be disabled and
+/// will not react to touch.
 ///
 /// See also:
 ///

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

@@ -25,8 +25,8 @@ import 'tooltip.dart';
 /// Icon buttons are commonly used in the [AppBar.actions] field, but they can
 /// be used in many other places as well.
 ///
-/// If the [onPressed] callback is not specified or null, then the button will
-/// be disabled, will not react to touch.
+/// If the [onPressed] callback is null, then the button will be disabled and
+/// will not react to touch.
 ///
 /// Requires one of its ancestors to be a [Material] widget.
 ///

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

@@ -18,11 +18,10 @@ import 'theme.dart';
 /// in long busy lists of content, or in wide spaces. Avoid using raised buttons
 /// on already-raised content such as dialogs or cards.
 ///
-/// If the [onPressed] callback is not specified or null, then the button will
-/// be disabled and by default will appear like a flat button in the
-/// [disabledColor]. If you are trying to change the button's [color] and it is
-/// not having any effect, check that you are passing a non-null [onPressed]
-/// handler.
+/// If the [onPressed] callback is null, then the button will be disabled and by
+/// default will appear like a flat button in the [disabledColor]. If you are
+/// trying to change the button's [color] and it is not having any effect, check
+/// that you are passing a non-null [onPressed] handler.
 ///
 /// Requires one of its ancestors to be a [Material] widget.
 ///