fix: allow empty typed slice flag values

[happysnaker]

Summary

• treat explicit empty values for typed comma-separated slice flags as empty slices instead of a single empty-string element
• keep the behavior aligned across IntSlice, Int32Slice, Int64Slice, UintSlice, Float32Slice, Float64Slice, and DurationSlice
• add regression coverage showing --flag= now produces a non-nil empty slice while leaving default values unchanged when the flag is omitted

Context

StringSlice already treats --flag= as an empty slice, but the typed slice implementations still fed the empty string through strings.Split, which turns it into []string{""} and then fails during numeric / duration parsing.

This change uses the same empty-input interpretation consistently for the typed slice variants, matching the expectation discussed in #222.

Notes

• UintSlice keeps the existing base-autodetection behavior from fix: allow hex input in UintSlice #484 while sharing the same empty-input helper

Closes #222

[happysnaker]

Maintainer context for review:

• this intentionally matches the collaborator guidance in Can't pass empty int slices #222: an explicit --flag= now becomes a non-nil empty slice, while omitting the flag still preserves the existing default value
• scope is limited to the typed comma-separated slice flags that still errored on the empty-string split path (IntSlice, Int32Slice, Int64Slice, UintSlice, Float32Slice, Float64Slice, DurationSlice)
• StringSlice already had this empty-value behavior, so the change is mainly about making the typed slice variants consistent instead of introducing a brand-new flag syntax

[tomasaschan]

Same as #486.