button [nfc]: Make onPressed required

All callers pass an `onPressed`, and the behavior when it's not
passed isn't coherent -- some aspects of the button take on a
disabled appearance following Material defaults (e.g. the background
is faded), but some don't, notably the icon color if an icon is
present.

Author: chrisbobbe

lib/widgets/button.dart

@@ -291,15 +291,15 @@ class ZulipMenuItemButton extends StatelessWidget {
     this.style = ZulipMenuItemButtonStyle.menu,
     required this.label,
     this.subLabel,
-    this.onPressed,
+    required this.onPressed,
     this.icon,
     this.toggle,
   });
 
   final ZulipMenuItemButtonStyle style;
   final String label;
   final TextSpan? subLabel;
-  final VoidCallback? onPressed;
+  final VoidCallback onPressed;
   final IconData? icon;
 
   /// A [Toggle] to go before [icon], or in its place if it's null.