Support for matrices/frames with zero rows and/or columns

[daphne-eu]

In GitLab by @pdamme on Sep 14, 2021, 22:12

Our current implementation implicitly assumes that a matrix/frame has at least one row and at least one column. However, this is not always true, e.g. the result of a filter could be empty (no rows). So we should generally allow matrices/frames without rows/columns. This might require changes in several places, starting at the data structures themselves.