Fix exported workspaces in ALC

[raquelalvarezbanos]

Fixes #11711

For tester:

• Code review: make sure you understand the problem reported by scientists (see original trac issue and documentation) and the fix.
• Reproduce the problem on the master branch: open ALC, load "MUSR00015189.nxs" as "First" and "MUSR00015193.nxs" as "Last", hit "Load", go to "Baseline Modelling", right-click on the "Function" blank region and add a LinearBackground, right-click on "Sections" blank section and add a section, hit "Fit", hit "Export results...", a WorkspaceGroup will be created. Right-click on "ALCResults_Baseline_Workspace" -> "Plot spectrum with errors" -> enter "0-1". Errors are huge for the "Baseline" curve.
• On this branch, repeat the same steps. Check that errors in "Baseline" are reasonable.