l1b_data should expect to be empty

[j-haacker]

l1b_data can be empty for different reasons. This can raise errors and stop pipelines.

If for some reason l1b_data is empty, still all parts of the code should execute and return the expected results.

Other packages use a boolean property .empty or similar. This could offer a straight forward and easily readable way. A wrapper function testing the input would be another option.