FIX: prevent error on init of SpikeData with empty ndarray

[dfsp-spirit]

Miny PR, fixes #257

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[codecov]

Codecov Report

Merging #317 (6a4a85a) into dev (ee9aa59) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #317      +/-   ##
==========================================
+ Coverage   72.58%   72.66%   +0.07%     
==========================================
  Files          72       72              
  Lines        7931     7931              
  Branches     1612     1612              
==========================================
+ Hits         5757     5763       +6     
+ Misses       1782     1779       -3     
+ Partials      392      389       -3     

Impacted Files	Coverage Δ
syncopy/datatype/discrete_data.py	83.33% <100.00%> (ø)
syncopy/datatype/base_data.py	82.47% <0.00%> (+0.20%)	⬆️
syncopy/shared/computational_routine.py	82.50% <0.00%> (+0.55%)	⬆️
syncopy/shared/tools.py	76.56% <0.00%> (+3.12%)	⬆️

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[tensionhead]

Thx for fixing this @dfsp-spirit !