apm-server panic with minimal config

**APM Server version** (`apm-server version`): https://github.com/elastic/apm-server/commit/e4649c0c38df7d72d417a671f0a44305c17595c5

**Description of the problem including expected versus actual behavior**:

**Steps to reproduce**:

config.yml:

```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/go-ucfg@v0.8.8/error.go:161 +0xc5
github.com/elastic/go-ucfg.raiseNil(...)
github.com/elastic/go-ucfg@v0.8.8/error.go:279
github.com/elastic/go-ucfg.(*Config).Unpack(0x0, {0x1d3c480, 0xc0000fe808}, {0xc000411980?, 0x0?, 0x0?})
github.com/elastic/go-ucfg@v0.8.8/reify.go:141 +0x15f
github.com/elastic/elastic-agent-libs/config.(*C).Unpack(...)
github.com/elastic/elastic-agent-libs@v0.10.1/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/cobra@v1.8.1/command.go:985 +0xaaa
github.com/spf13/cobra.(*Command).ExecuteC(0xc0004f2008)
github.com/spf13/cobra@v1.8.1/command.go:1117 +0x3ff
github.com/spf13/cobra.(*Command).Execute(...)
github.com/spf13/cobra@v1.8.1/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
```