When theres no ROIs, disable fitting buttons and plot empty spectrum

[MikeSullivan7]

Issue Closes #3123

Description

There are now checks when the Fitting Tab is activated so that when we are in ROI mode and all ROIs have been deleted from the table, an empty spectrum is plotted and the "From ROI" and "Run All Fit" buttons are disabled as there is no data to fit against.

Developer Testing

• I have verified unit tests pass locally: python -m pytest -vs
• ...

Acceptance Criteria and Reviewer Testing

• Unit tests pass locally: python -m pytest -vs
• Load in data and open Spectrum Viewer
• With an ROI activate, open the Fitting Tab and check that it behaves as normal.
• Go back to the Image Tab and delete the ROI so that the table is completely empty
• Go to the Fitting Tab and check that the ROI Selection Widget is empty, and that the relevant buttons are disabled.
• Add an ROI and go back to the Fitting Tab, check that everything is enabled as usual.

[coveralls]

coverage: 67.959% (-0.04%) from 67.998% — 3123_no_rois_fitting_tab_keyerror into main

[ashleymeigh2]

KeyError occurs here because no ROI is selected in the fitting form dropdown.
current_roi_name is empty ('') at this point, so roi_dict lookup fails.

[RasmiaKulan]

looks good