Allow index IsDescending to get empty array for all-descending

[roji]

Improves #27210 by allowing aan empty list parameter to IsDescending, meaning that all index columns should be descending.
Also normalizes explicitly-set all-ascending/all-descending to null/empty array respectively.

(addresses API review, #27588)

[roji]

Question: is it worth introducing AllDescending or similar on IndexAttribute? Right now to specify all-descending, you do this ugly thing:

[Index(..., IsDescending = new bool[0])]

[AndriySvyryd]

I'd prefer adding AllDescending to giving special meaning to an empty array
@dotnet/efteam

[roji]

@dotnet/efteam added AllDescending.