chore(lint): fix some linting complains

[ricardomaraschini]

Description

this addresses the following complains i found running the lint locally:

pkg/apis/k0s/v1beta1/storage.go:276:5: S1009: should omit nil check; len() for []string is defined as zero (gosimple)
        if e.Endpoints == nil || len(e.Endpoints) == 0 {
           ^
internal/pkg/iptablesutils/iptables_test.go:215:1: SA3000: TestMain should call os.Exit to set exit code (staticcheck)
func TestMain(m *testing.M) {
^
inttest/cli/cli_test.go:64:10: printf: non-constant format string in call to (*testing.common).Logf (govet)
                t.Logf(out)
                       ^

Type of change

• [x] Chore
• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation update

How Has This Been Tested?

• [ ] Manual test
• [ ] Auto test added

Checklist:

• [x] My code follows the style guidelines of this project
• [x] My commit messages are signed-off
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules
• [x] I have checked my code and corrected any misspellings