Fix stupid sparse bugs with outerSize == 0

Reference issue

What does this implement/fix?

Calling nonZeros() on an empty sparse matrix could segfault since m_outerIndex is null. There are many more cases where this could be an issue: makeCompressed, the various flavors of insert, the list goes on. Reverted change so that a sparse matrix with outerSize == 0 allocates a size-1 array.

Additional information