Update documentation of Object3D#userData (mrdoob#26799)

* Update userData description in Object3D.html

Updated the documentation to make clear what the default value is (otherwise one could assume it might be null or undefined).

* Update Texture.html

* Update Material.html

* Update BufferGeometry.html

docs/api/en/core/BufferGeometry.html

@@ -191,6 +191,7 @@ <h3>[property:Object userData]</h3>
 		<p>
 			An object that can be used to store custom data about the BufferGeometry.
 			It should not hold references to functions as these will not be cloned.
+			Default is an empty object `{}`.
 		</p>
 
 		<h3>[property:String uuid]</h3>

docs/api/en/core/Object3D.html

@@ -206,6 +206,7 @@ <h3>[property:Object userData]</h3>
 		<p>
 			An object that can be used to store custom data about the Object3D. It
 			should not hold references to functions as these will not be cloned.
+			Default is an empty object `{}`.
 		</p>
 
 		<h3>[property:String uuid]</h3>

docs/api/en/materials/Material.html

@@ -369,6 +369,7 @@ <h3>[property:Object userData]</h3>
 		<p>
 			An object that can be used to store custom data about the Material. It
 			should not hold references to functions as these will not be cloned.
+			Default is an empty object `{}`.
 		</p>
 
 		<h2>Methods</h2>

docs/api/en/textures/Texture.html

@@ -281,6 +281,7 @@ <h3>[property:Object userData]</h3>
 		<p>
 			An object that can be used to store custom data about the texture. It
 			should not hold references to functions as these will not be cloned.
+			Default is an empty object `{}`.
 		</p>
 
 		<h3>[property:Source source]</h3>