LMarker: if icon passed is not an acceptable LeafletIcon override it

[DonNicoJs]

Currently if in the prop icon is passed something that is not a valid leaflet icon instance it would break the code.
We need to change this, if the user pass something which is not an instance of L.icon (https://leafletjs.com/reference-1.3.4.html#icon) we should override it with new L.Icon.Default()

marking this issue for the hackotcoberfest

[stale]

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