Add explicit cast to period.count() (backport #1404)

[mergify]

period.count() does not return an integer.
Fails with changes from ros-controls/control_toolbox#239 otherwise.

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This is an automatic backport of pull request #1404 done by Mergify.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 66.31%. Comparing base (b133273) to head (15359e0).
Report is 1 commits behind head on humble.

Files with missing lines	Patch %	Lines
...gripper_controllers/hardware_interface_adapter.hpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           humble    #1405      +/-   ##
==========================================
+ Coverage   66.27%   66.31%   +0.03%     
==========================================
  Files         110      110              
  Lines       12662    12662              
  Branches     7812     7815       +3     
==========================================
+ Hits         8392     8397       +5     
- Misses       1409     1415       +6     
+ Partials     2861     2850      -11     

Flag	Coverage Δ
unittests	66.31% <0.00%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...gripper_controllers/hardware_interface_adapter.hpp	56.66% <0.00%> (ø)

... and 4 files with indirect coverage changes