fix: improve error message when no benchmarks are found

[GuillaumeLagrange]

Handle the NoBenchmarks variant in ReportConclusion and display a warning when results are empty instead of silently showing nothing.

[codspeed-hq]

Merging this PR will not alter performance

✅ 4 untouched benchmarks

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_{Comparing cod-2158-improve-error-message-when-no-benchmarks-with-exec-cli (fd73399) with main (7cd7a64)}