FIX: Scale transformation fails if passed constant input.

[tyarkoni]

Closes #578

[codecov]

Codecov Report

Merging #614 into master will increase coverage by 0.76%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #614      +/-   ##
==========================================
+ Coverage   83.63%   84.39%   +0.76%     
==========================================
  Files          25       25              
  Lines        3196     3320     +124     
  Branches      810      886      +76     
==========================================
+ Hits         2673     2802     +129     
+ Misses        335      331       -4     
+ Partials      188      187       -1     

Flag	Coverage Δ
#unittests	84.39% <100.00%> (+0.76%)	⬆️

Impacted Files	Coverage Δ
bids/analysis/transformations/compute.py	88.79% <100.00%> (+2.06%)	⬆️
bids/layout/layout.py	88.31% <0.00%> (+2.94%)	⬆️

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