Allow "None" as modifier string for bindings without modifiers

This is more userfriendly than the empty string approach.

doc/riverctl.1.scd

@@ -153,8 +153,7 @@ that tag 1 through 9 are visible.
 	_key_ is an XKB key name. See _/usr/include/xkbcommon/xkbcommon-keysyms.h_
 	for a list of special key names. _command_ can be any of the above commands.
 
-	A mapping without modifiers can be created by passing an empty string as
-	the modifiers argument.
+	A mapping without modifiers can be created by using "None" as sole modifier.
 
 *map-pointer* _mode_ _modifiers_ _button_ _action_
 	_mode_ and _modifiers_ are the same as for *map*.

river/command/map.zig

@@ -29,7 +29,7 @@ const modifier_names = [_]struct {
     name: []const u8,
     modifier: u32,
 }{
-    .{ .name = "", .modifier = 0 },
+    .{ .name = "None", .modifier = 0 },
     .{ .name = "Shift", .modifier = c.WLR_MODIFIER_SHIFT },
     .{ .name = "Lock", .modifier = c.WLR_MODIFIER_CAPS },
     .{ .name = "Control", .modifier = c.WLR_MODIFIER_CTRL },