Fix: map.compute_centroids fail for maps with no fragments

[AhmetNSimsek]

import siibra
mp = siibra.get_map('julich 3', 'mni152')
mp.compute_centroids()

expected: computes the centroids using the volumes of the map.
actual: cannot fetch the mask

This PR fixes this issue

[codecov-commenter]

Codecov Report

Merging #486 (b5d64cd) into main (5a0e6b3) will increase coverage by 0.62%.
Report is 149 commits behind head on main.
The diff coverage is 27.07%.

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@@            Coverage Diff             @@
##             main     #486      +/-   ##
==========================================
+ Coverage   36.81%   37.44%   +0.62%     
==========================================
  Files          61       61              
  Lines        5421     5651     +230     
==========================================
+ Hits         1996     2116     +120     
- Misses       3425     3535     +110     

Files	Coverage Δ
siibra/configuration/configuration.py	42.35% <ø> (ø)
siibra/core/atlas.py	85.71% <ø> (+1.23%)	⬆️
siibra/core/region.py	45.61% <100.00%> (+1.31%)	⬆️
siibra/features/connectivity/__init__.py	83.33% <100.00%> (ø)
...ibra/features/connectivity/tracing_connectivity.py	75.00% <100.00%> (ø)
siibra/features/image/__init__.py	100.00% <100.00%> (ø)
siibra/features/tabular/layerwise_cell_density.py	37.20% <ø> (+0.25%)	⬆️
siibra/locations/boundingbox.py	19.88% <ø> (ø)
...a/features/connectivity/functional_connectivity.py	55.55% <0.00%> (ø)
siibra/features/image/volume_of_interest.py	68.42% <66.66%> (-0.33%)	⬇️
... and 29 more

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