add_bsplines_operator() broken for `m1_bs0` operator=`'D1N2'`

[Didou09]

See https://github.com/ToFuProject/tofu/actions/runs/17735605259/job/50396502825?pr=1126

Operator matrix has full-zero columns / lines
Causing inversion matrix to be singular => inversion fails

import tofu as tf
test = tf.tests.tests07_inversions.test_01_isotropic.Test01_Inversions()
test.setup_class()
test.setup_method()

# get operator
dout = test.coll.add_bsplines_operator(store=False, operator='D1N2', key='m1_bs0')

# plot operator
plt.figure()
plt.imshow((dout['tMM1']['data'] + dout['tMM0']['data']).todense(), aspect='equal', interpolation='nearest')
plt.colorbar()

# to get debug figure
test.test01_run_all_and_plot(debug=True)

Should be similar to :