keybind: don't clobber the clipboard if the title is empty

src/Surface.zig

@@ -4445,6 +4445,7 @@ pub fn performBindingAction(self: *Surface, action: input.Binding.Action) !bool
 
         .copy_title_to_clipboard => {
             const title = self.rt_surface.getTitle() orelse return false;
+            if (title.len == 0) return false;
 
             self.rt_surface.setClipboardString(title, .standard, false) catch |err| {
                 log.err("error copying title to clipboard err={}", .{err});

src/input/Binding.zig

@@ -282,7 +282,7 @@ pub const Action = union(enum) {
     copy_url_to_clipboard,
 
     /// Copy the terminal title to the clipboard. If the terminal title is not
-    /// set this has no effect.
+    /// set or is empty this has no effect.
     copy_title_to_clipboard,
 
     /// Increase the font size by the specified amount in points (pt).