fix: allow explicit empty numeric slice values

[sb123sb123]

Fixes #222

Passing --data= to numeric and duration slice flags currently attempts to parse an empty token, so callers cannot distinguish an explicitly requested empty slice from an omitted flag.

This change treats the first explicit empty value as a non-nil empty slice for duration, float32, float64, int, int32, int64, and uint slices. It preserves nil, empty, and populated defaults when the flag is omitted, and keeps repeated non-empty values appendable. Bool, string, IP, and CIDR slice types already accept this syntax.

A matching earlier PR (#512) was withdrawn by its author because the required CLA could not be completed; the issue remains open and this change is independently tested against the current master base.

Tests:

• go test . -run "Test.*Slice" -count=1 with Go 1.26.7
• go vet . with Go 1.26.7
• The added regression failed on the pre-change master base for duration, float32, float64, int, int32, int64, and uint with empty-token parse errors, then passed after the fix.

Limitations: the full repository test suite, GitHub Actions matrix, and lint workflow were not run locally.

AI assistance: OpenAI Codex (GPT-5.6 Luna).

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