fix: small fix and flake8

Description

Changes Made

• Account for nans by using np.nan(percentile, min, max, mean, median)
• Check for errors when all of the data is nans. https://redmine.xfel.eu/issues/208976 multiple cells had constants data value of NANs

Screenshots

Related Issues

• Issue: https://git.xfel.eu/calibration/calephant/-/issues

Tests

Additional Notes

Merge Request Checklist

• Code follows project coding guidelines.
• Documentation reflects the changes made.
• Unit tests are added or updated to cover the changes.
• The application runs without errors and the new functionality works as expected.