URL properties not cleared after reset

When resetting a URL property (using the cog context menu in Property, Material, or Texture Editor), the instance value still contains the old value before the reset.

Expectation:

After resetting a url property, a call like below should return empty:

QmlObjectNode(m_selectedTexture).instanceValue(property.name()); // should be empty

Currently this is worked around by resetting the puppet.