Remove redundant nil check in LocalFlagNames

[Juneezee]

What type of PR is this?

• cleanup

What this PR does / why we need it:

From the Go specification:

"1. For a nil slice, the number of iterations is 0." https://go.dev/ref/spec#For_range

Therefore, an additional nil check for before the loop is unnecessary. Example: https://go.dev/play/p/_cXfzMOvMm3

Testing

go test ./...
?   	github.com/urfave/cli/v3/internal/build	[no test files]
?   	github.com/urfave/cli/v3/internal/example-cli	[no test files]
?   	github.com/urfave/cli/v3/internal/example-hello-world	[no test files]
ok  	github.com/urfave/cli/v3	0.101s

Release Notes

NONE

[github-actions]

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