[REMI] Make plots robuster against missing signal

Description

I feel like I did this already, but it continues to happen - at least one instance affects a plot added since then, so I likely forgot.

Depending on experimental needs or signal conditions, it is not extraordinary that one of the REMI detectors may have no detectable hits for a run or two. As agreed with the instrument, it is more desirable to not error out in this case but print visible warnings. This should still catch sufficiently the less likely case of unintended missing signal while ensuring normal operation for the other detector.

Since this is happening a lot this week, I will hotfix the notebook immediately with this MR.

How Has This Been Tested?

This run has signal on the bottom detector but none on the top detector.

run = 88  # Run ID.
in_folder = '/gpfs/exfel/exp/SQS/202401/p005680/raw'  # Partial input path appended with run ID.
calib_config_path = '/gpfs/exfel/exp/SQS/202401/p005680/usr/remi_proc_config.yaml'  # Path to correction and transform configuration

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.

Reviewers

@ahmedk