Fix add_widget for undefined roi

[ericpre]

Improvement of #2341.

Progress of the PR

• Add support for "undefined" ROI in add_widget
• add tests,
• ready for review.

Minimal example of the bug fix or the new feature

import hyperspy.api as hs

im = hs.datasets.example_signals.object_hologram()
im.plot()

roi = hs.roi.Line2DROI()
roi.add_widget(im)

Previously, only roi.interactive(s) would work.

[codecov]

Codecov Report

Merging #2767 (94d87d2) into RELEASE_next_minor (384b4e6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@                 Coverage Diff                 @@
##           RELEASE_next_minor    #2767   +/-   ##
===================================================
  Coverage               78.00%   78.01%           
===================================================
  Files                     203      203           
  Lines                   30695    30702    +7     
  Branches                 6698     6704    +6     
===================================================
+ Hits                    23945    23952    +7     
  Misses                   4995     4995           
  Partials                 1755     1755           

Impacted Files	Coverage Δ
hyperspy/drawing/_widgets/line2d.py	54.29% <100.00%> (ø)
hyperspy/roi.py	84.48% <100.00%> (+0.16%)	⬆️

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