Wrong dim of linalg degree when no neighbours

[maximelucas]

Running

import xgi

edges = [[0,1], [0,2], [1,2,3]]
H = xgi.Hypergraph(edges)

yields

xgi.degree(H, order=3).shape
## --> (0,)

but it should be (4,), the number of nodes.

This then yields an error when running xgi.laplacian(H, order=3), because it conflicts with the (correct) dimension of the adjacency matrix:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Input In [16], in <cell line: 1>()
----> 1 xgi.laplacian(H, order=3).shape

File ~/Dropbox (ISI Foundation)/WORK/SCIENCE/xgi/xgi/linalg/matrix.py:274, in laplacian(H, order, rescale_per_node, index)
    270     return (np.array([]), {}) if index else np.array([])
    272 K = degree(H, order=order, index=False)
--> 274 L = order * np.diag(np.ravel(K)) - A  # ravel needed to convert sparse matrix
    275 L = np.asarray(L)
    277 if rescale_per_node:

ValueError: operands could not be broadcast together with shapes (0,0) (4,4)