Fixed issue with loading all-zero sparse matrices, as the data and ir…

[SebastiaanBreedveld]

… pointers are not stored in the .mat file.

[skjerns]

perfect!

bonus: could you provide a sample file for this case and write a test in https://github.com/skjerns/mat7.3/blob/master/tests/test_mat73.py ?

alternatively, you can just send me a sample file or some script to create a sample file in MATLAB and I can add that later.

[SebastiaanBreedveld]

Sorry, I understand the importance of unit tests, but this is time-wise beyond my reach for now. I also have not used this package directly but this issue caused a problem with another script.

It should be reproducible by using the following Matlab code:
A = sparse([0 0 0; 0 0 0]);
save -v7.3 testSparse.mat A

[skjerns]

sure, no worries! the MATLAB code is enough for me to implement it :)

[skjerns]

thanks @SebastiaanBreedveld !

I've added unittests in #58