For the time being you can either declare some command or place the following JavaScript workaround below the RadDock's declaration:
		<telerik:RadDockZone ID="RadDockZone1" runat="server" Height="100%" MinWidth="50">
			<telerik:RadDock ID="RadDock1" runat="server" RenderMode="Lightweight" Title="title"
				DefaultCommands="None" DockHandle="TitleBar" DockMode="Docked">
				<ContentTemplate>
					<div>
						Content...
					</div>
				</ContentTemplate>
			</telerik:RadDock>
		</telerik:RadDockZone>
		<script>
			Telerik.Web.UI.Dock.LightweightView.prototype._sizeTitleElement = function () {
				var titleElem = this.get_titleElement(),
					titleBarElem = this.get_titleBar(),
					commandsElem = this.get_commandsContainer();
				if (commandsElem == undefined) {
					titleElem.style.width = Math.max(0, parseInt(titleBarElem.clientWidth)) + "px";
				}
				else {
					titleElem.style.width = Math.max(0, parseInt(titleBarElem.clientWidth) - parseInt(commandsElem.clientWidth)) + "px";
				}
				
			}
		</script>