Clear regex output variables

[thorinaboenke]

Task

#255

Description

Fix

• register_outputvars: on no-match, clear each declared output variable to '' before returning.
• search no-match branch now routes through forge_and_register_variables(command.output, None) --> same clearing path.

Only the variables named in that command's own output: block are affected. No other variables in the VariableStore are touched

Tests
Updated findall/search no-match tests to expect the cleared '' value.

• Added test_exec_cmd_search_no_match_clears_stale_value reproducing the loop scenario.

Checklist

• This Pull-Request goes to the development branch.
• I have successfully run prek locally.
• I have added tests to cover my changes.
• I have linked the issue-id to the task-description.
• I have performed a self-review of my own code.