Fixing constructors from 0-element arrays

[mateuszbaran]

Maybe not the prettiest solution but still fixes #557 and #782.

[mateuszbaran]

That failure looks like a Julia 1.0 bug and I don't see an obvious way to work around it 😕 .

[mateuszbaran]

Apart from not solving the problem of converting empty StaticArrays to SizedArray or SizedMatrix on Julia 1.0, this seems to work now 🙂 . Anyway, I think these two cases can be fixed in another PR if someone considers it worth fixing.

[c42f]

Ack, sorry for the delay in replying to this. I'm a little unsure about this solution in the sense that it requires a new constructor for each StaticArray subtype. I feel like that shouldn't be necessary...

However, I also know that the generic dispatch for StaticArray constructors is quite hairy and hard to modify and that this does fix the bugs. So I think on balance we should just go with it.

(Regarding 1.0 - at this point it's basically EOL and I'm looking forward to removing support for it once 1.6 (or 1.7?) is declared the new LTS. So I'm not too worried about that.)

[mateuszbaran]

I'm a little unsure about this solution in the sense that it requires a new constructor for each StaticArray subtype. I feel like that shouldn't be necessary...

After working with this a bit I feel that a part of hairiness of static array constructors is trying to define things for all subtypes of StaticArray. But I may be wrong. At the very least not all subtypes of StaticArray are constructible just from elements.