changelog: Do not add to a nil map

[jrajahalme]

Initialize a nil map before writing to it.

From https://github.com/cilium/cilium/actions/runs/10349027982/job/28642554669:

panic: assignment to entry in nil map

goroutine 1 [running]: github.com/cilium/release/cmd/changelog.(ChangeLog).PrintReleaseNotesForWriter(0xc0005f6270, {0xb0e140, 0xc0003a4090}) /home/runner/work/cilium/release/cmd/changelog/generate.go:181 +0x170e github.com/cilium/release/cmd/release.(PrepareCommit).generateChangeLog(0xc000058128, {0xb12260, 0xc00007cb90}, 0xc0001e2b90) /home/runner/work/cilium/release/cmd/release/git.go:244 +0x40c github.com/cilium/release/cmd/release.(PrepareCommit).Run(0xc000058128, {0xb12260, 0xc00007cb90}, 0x5f?, 0x25?, 0xc0001e2b90) /home/runner/work/cilium/release/cmd/release/git.go:139 +0xc65 github.com/cilium/release/cmd/release.Command.func2(0xc000193208, {0xa39d47?, 0x4?, 0xa39d4b?}) /home/runner/work/cilium/release/cmd/release/start.go:288 +0x98c github.com/spf13/cobra.(Command).execute(0xc000193208, {0xc00011e840, 0xb, 0xb}) /home/runner/work/cilium/release/vendor/github.com/spf13/cobra/command.go:983 +0xaca github.com/spf13/cobra.(Command).ExecuteC(0xefc780) /home/runner/work/cilium/release/vendor/github.com/spf13/cobra/command.go:1115 +0x3ff github.com/spf13/cobra.(Command).Execute(...) /home/runner/work/cilium/release/vendor/github.com/spf13/cobra/command.go:1039 main.main() /home/runner/work/cilium/release/cmd/main.go:115 +0x1a