Add warning message "No input data" for commandline tool when there are NO allure-result files at input directory

**Is your feature request related to a problem? Please describe.**
As a Allure-newcomer I got 3 hours headache trying to understand why I see broken Allure-report in my browser (with "NaN" and "unknown" words). Problem was that I wrongfully understand what exactly `serve` do and point it to wrong directory without allure-results. But because of silent "normal" working of **commandline tool** - I thought, that problem is with my "badly" generated json-results files...

**Describe the solution you'd like**
Add at least WARNING MESSAGE to console, when **commandline tool** COULD NOT FIND any of appropriate input formats (for me it was json files). In other words - there should be message to poor user, which indicates, that tool now generating BROKEN/EMPTY report.

The issue does not name files, tests, or a specific entry point. Start by locating the command-line path that processes an input directory, then verify its behavior when no Allure result files are present. Done means the tool emits a warning that the report may be empty or broken.

Written by the indexing model from the issue text.