_io.concatenate()` may fail if extracted laison cube is scalar

[valeriupredoi]

Raised (inadvertently) by @bouweandela in this comment - it all happens in _io._concatenate_overlapping_cubes(cubes) - after extract_time() the resulting cube may be scalar - so iris concatenation fails due to scalar cubes not having dim_metadata attributes in iris._concatenate_CubeSignature class (they do, but they are empty lists).

Now. my idea would be this to be fixed in extract_time() directly, but for now I'll fix it in our concatenation, not to belly up other preprocessors that use extract_time().

Scalar cubes are PITAs!

Fixed in #1715