NVTX: null destination pointer warning-treated-as-error

[achirkin]

Some compiler versions think the buf can be empty in the definition of the nvtx push_range, resulting in an error:

raft/core/detail/nvtx.hpp:169:14: error: null destination pointer [-Werror=format-truncation=]
  169 |     std::snprintf(buf.data(), length + 1, format, args...);

This fix gives static guarantees to the compiler that the temporary buf length is positive.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.83%. Comparing base (8f79e34) to head (c7181b4).

Additional details and impacted files

@@              Coverage Diff              @@
##           branch-25.06    #2639   +/-   ##
=============================================
  Coverage         80.83%   80.83%           
=============================================
  Files                19       19           
  Lines               527      527           
=============================================
  Hits                426      426           
  Misses              101      101           

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[benfred]

/merge