title: How to flatten specific group in linq and select all elements, and only first from other groups
body: <p>I have a list of items, where some items may have null/zero values of specific item's property. I am grouping items by other property. Then all I want, is to select First() element of those groups where my property != null/zero, and all the items in group where that property equals null/zero. How to do that?</p>