IOTData: Fix delta calculation

[bblommers]

Fixes #7695

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.34%. Comparing base (1f3fd9e) to head (4f521ba). Report is 8 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7814   +/-   ##
=======================================
  Coverage   94.34%   94.34%           
=======================================
  Files        1107     1107           
  Lines       93980    94009   +29     
=======================================
+ Hits        88663    88693   +30     
+ Misses       5317     5316    -1     

Flag	Coverage Δ
servertests	29.33% <8.69%> (+<0.01%)	:arrow_up:
unittests	94.31% <100.00%> (+<0.01%)	:arrow_up:

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[github-actions[bot]]

This is now part of moto >= 5.0.11.dev15