[2D] [Animation] Error appears in console when trying to set SpriteLibrary Category List size to negative value

Steps to Repro:

1. Create a Sprite Library Asset
2. Create a new Category
3. Click over the Size field and move the mouse to the left (to try to set the Size value to less than 0)

Result: An Error appears in the console.
Expected: The user should be unable to set the value to a negative
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Array size must be non-negative.
UnityEditor.Experimental.U2D.Animation.SpriteLibraryAssetInspector:DrawElement(Rect, Int32, Boolean, Boolean)
UnityEditorInternal.ReorderableList:DoListElements(Rect)
UnityEditorInternal.ReorderableList:DoLayoutList()
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)