[BUG] gazebo citadel fails to load sdf files that aren't a part of a <world> but doesn't throw an error.

[rydb]

Environment

• OS Version: Ubuntu 20.04
  binary build
  Citadel v3.14.0

Description

• Expected behavior: gazebo throws an error to state that sdfs must be part of a world to be loaded, then closes
• Actual behavior: gazebo says that it load the file, then gazebo terminates it self with no error.

Steps to reproduce

• generate an sdf from with the urdf -> sdf converter tool via `ign sdf -p"
• attempt the load the sdf via `ign gazebo -v4 <path_to_sdf>

Output

[Msg] Ignition Gazebo GUI    v3.14.0
[Dbg] [ign.cc:160] Subscribing to [/gazebo/starting_world].
[Dbg] [ign.cc:162] Waiting for a world to be set from the GUI...
[Dbg] [Gui.cc:245] Waiting for subscribers to [/gazebo/starting_world]...
[Msg] Received world [<path_to_sdf>/diff_bot.sdf] from the GUI.
[Dbg] [ign.cc:166] Unsubscribing from [/gazebo/starting_world].
[Msg] Ignition Gazebo Server v3.14.0
[Msg] Loading SDF world file[<path_to_sdf>diff_bot.sdf].
[Msg] Serving world names on [/gazebo/worlds]
[Msg] Resource path add service on [/gazebo/resource_paths/add].
[Msg] Resource path get service on [/gazebo/resource_paths/get].
[Msg] Resource paths published on [/gazebo/resource_paths].
[Msg] Server control service on [/server_control].
[Dbg] [ign.cc:410] Shutting down ign-gazebo-server