apm-server panic with minimal config

[kruskall]

APM Server version (apm-server version): e4649c0

Description of the problem including expected versus actual behavior:

Steps to reproduce:

config.yml:

output.elasticsearch:
  hosts: ["https://127.0.0.1:8443"]

  username: "admin"
  password: "changeme"

  ssl.verification_mode: none

./apm-server -e run -c config.yml

Provide logs (if relevant):

runtime/debug.Stack()
	runtime/debug/stack.go:26 +0x5e
github.com/elastic/go-ucfg.raiseCritical({0x2541b20, 0x3919e20}, {0x0?, 0x1?})
	github.com/elastic/[email protected]/error.go:161 +0xc5
github.com/elastic/go-ucfg.raiseNil(...)
	github.com/elastic/[email protected]/error.go:279
github.com/elastic/go-ucfg.(*Config).Unpack(0x0, {0x1d3c480, 0xc0000fe808}, {0xc000411980?, 0x0?, 0x0?})
	github.com/elastic/[email protected]/reify.go:141 +0x15f
github.com/elastic/elastic-agent-libs/config.(*C).Unpack(...)
	github.com/elastic/[email protected]/config/config.go:159
github.com/elastic/apm-server/internal/beater/config.NewConfig(0x0, 0xc0004fc2d0)
	github.com/elastic/apm-server/internal/beater/config/config.go:88 +0x8c
github.com/elastic/apm-server/internal/beater.NewRunner({0xc0004fc120?, 0xc0002c2ab0?, 0x22f3b78?})
	github.com/elastic/apm-server/internal/beater/beater.go:126 +0xc5
main.Main.func1({0xc0004fc120, {{0x21e4264, 0xa}, {0x21e4264, 0xa}, {0x21dcacf, 0x5}, 0x1, {0xc00042723a, 0x6}, ...}, ...})
	github.com/elastic/apm-server/x-pack/apm-server/main.go:280 +0x27
github.com/elastic/apm-server/internal/beatcmd.(*Beat).Run(0xc00027ea80, {0x2557530, 0xc0004ec780})
	github.com/elastic/apm-server/internal/beatcmd/beat.go:384 +0x95c
github.com/elastic/apm-server/internal/beatcmd.genRunCmd.func1(0xc0001f4a00?, {0x21dc036?, 0x4?, 0x21dc03a?})
	github.com/elastic/apm-server/internal/beatcmd/run.go:47 +0xae
github.com/spf13/cobra.(*Command).execute(0xc0001ea608, {0xc00038e8d0, 0x3, 0x3})
	github.com/spf13/[email protected]/command.go:985 +0xaaa
github.com/spf13/cobra.(*Command).ExecuteC(0xc0004f2008)
	github.com/spf13/[email protected]/command.go:1117 +0x3ff
github.com/spf13/cobra.(*Command).Execute(...)
	github.com/spf13/[email protected]/command.go:1041
main.Main()
	github.com/elastic/apm-server/x-pack/apm-server/main.go:287 +0x32
main.main()
	github.com/elastic/apm-server/x-pack/apm-server/main.go:293 +0x13

[kruskall]

The issue seems to be the missing

apm-server:
  host: "127.0.0.1:8200"