[scanbase] keep original behavior: flat_field_correction always return a list...

[scanbase] keep original behavior: flat_field_correction always return a list of nuloy array even if flat field correction fails
4 jobs for rework_flat_field_correction in 3 minutes and 51 seconds (queued for 1 second)
Status Name Job ID Coverage
  Style
passed check_style #137424

00:00:15

 
  Test Library
passed test:python3.5-stretch-pyqt5 #137425

00:02:15

passed test:python3.7_stretch_pyqt5 #137426

00:02:07

 
  Test Notbooks
passed test:test-tomoscan-tutorials #137427

00:01:20