FMRILinearModel handles the case where contrasts are null for some fmri sessions
Syntax change in FMRILinearModel.contrast implementation
Added test in test_glm.test_high_level_glm_null_contrasts and fixed a glitch in test_glm.generate_fake_fmri_data
Removed useless contrast in test_glm.test_high_level_glm_null_contrasts
Merge branch 'glm_null_contrasts_rerebased' into main-master

* glm_null_contrasts_rerebased:
  Removed useless contrast in test_glm.test_high_level_glm_null_contrasts
  Added test in test_glm.test_high_level_glm_null_contrasts and fixed a glitch in test_glm.generate_fake_fmri_data
  Syntax change in FMRILinearModel.contrast implementation
  FMRILinearModel handles the case where contrasts are null for some fmri sessions