Allow empty spots on matrix multi-substation diagrams

[So-Fras]

Describe the current behavior

It is currently not possible to display 5 substations according to a 2 X 3 matrix (leaving a spot empty). If an empty String is used to show the empty spot, an exception is returned (because the substation "" has not been found in the network).

Describe the expected behavior

The user should be able to give an empty String to indicate an empty spot in the substation matrix.

Describe the steps

No response

Environment

pypowsybl v1.3.0

Relevant Log Output

No response

Extra Information

No response