Fix -Werror build of rpy_publisher

[CurryMonsters]

Passing {} as the default value to declare_parameter<std::vectorstd::string> triggers "converting to ParameterDescriptor from initializer list would use explicit constructor" under -Wall -Wextra -Werror, which CI uses. Spell out the default as std::vectorstd::string{}.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.86%. Comparing base (172b932) to head (3826fb6).

Files with missing lines	Patch %	Lines
vortex_utility_nodes/src/rpy_publisher.cpp	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #55      +/-   ##
==========================================
- Coverage   62.97%   62.86%   -0.11%     
==========================================
  Files          15       15              
  Lines        1723     1726       +3     
  Branches      819      819              
==========================================
  Hits         1085     1085              
- Misses        410      413       +3     
  Partials      228      228              

Flag	Coverage Δ
unittests	62.86% <0.00%> (-0.11%)	⬇️

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

Files with missing lines	Coverage Δ
vortex_utility_nodes/src/rpy_publisher.cpp	0.00% <0.00%> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.