Update get_image doc to mention that it will return an empty image with invalid texture

doc/classes/Texture2D.xml

@@ -123,6 +123,7 @@
 			<return type="Image" />
 			<description>
 				Returns an [Image] that is a copy of data from this [Texture2D] (a new [Image] is created each time). [Image]s can be accessed and manipulated directly.
+				[b]Note:[/b] This will return [code]null[/code] if this [Texture2D] is invalid.
 				[b]Note:[/b] This will fetch the texture data from the GPU, which might cause performance problems when overused.
 			</description>
 		</method>