Fix bug in apiserver service cidr split

[liggitt]

What type of PR is this?
/kind bug

Based on #85937 with #85937 (comment) addressed

What this PR does / why we need it:
Fix bug where kube-apiserver would fail to start when not providing service CIDR

Which issue(s) this PR fixes:
NA

Special notes for your reviewer:
The strings.Split documentation states:

// If s does not contain sep and sep is not empty, Split returns a
// slice of length 1 whose only element is s.

import (
	"fmt"
	"strings"
)

func main() {
	serviceClusterIPRanges := ""
	serviceClusterIPRangeList := strings.Split(serviceClusterIPRanges, ",")
	fmt.Printf("len: %d, value: %+v", len(serviceClusterIPRangeList), serviceClusterIPRangeList)

}

// Output
// len: 1, value: []

playground link

This would fail the if check here and apiserver wouldn't start. This PR fixes this bug.

Does this PR introduce a user-facing change?:
NA

Release note:

NONE

[BenTheElder]

/lgtm

[justaugustus]

Thanks for the carry, @liggitt!

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: justaugustus, liggitt

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• cmd/kube-apiserver/OWNERS [liggitt]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment