joinGiottoObjects() fails when polys but not expression is present

[jiajic]

When assembling the joined Giotto object for return, the expression information then the spatlocs are added.
Later, the spatial information and feature points are added.

This is fine for datasets with expression and spatlocs, but for datasets with only polygons and/or features, this violates the Giotto hierarchical checks.

A change in the order of data appending may resolve this.