Appending a Document to an empty Document Set panics

**What steps did you take:**

With this input (i.e. an overlay that appends a document):

```
#@overlay/match by=lambda: False
#@overlay/append
---
kind: Stuff
```

ran:

```bash
$ ytt -f .
```

**What happened:**

```bash
panic: runtime error: index out of range [-1]

goroutine 1 [running]:
github.com/k14s/ytt/pkg/yttlibrary/overlay.Op.appendDocument(...)
github.com/k14s/ytt/pkg/yttlibrary/overlay/document.go:195
github.com/k14s/ytt/pkg/yttlibrary/overlay.Op.applyDocSet({{0x1469760, 0xc00000cb70}, {0x14f1640, 0xc0001a8cb0}, 0xc0001ea9b0, 0x0}, {0x1993b70, 0x1a7c101, 0x0}, 0xc0001a8cb0, ...)
github.com/k14s/ytt/pkg/yttlibrary/overlay/op.go:155 +0x805
github.com/k14s/ytt/pkg/yttlibrary/overlay.Op.apply({{0x1469760, 0xc00000cb70}, {0x14f1640, 0xc0001a8cb0}, 0xc0001ea9b0, 0x0}, {0x1469760, 0xc00000cb88}, {0x14f1640, 0xc0001a8cb0}, ...)
github.com/k14s/ytt/pkg/yttlibrary/overlay/op.go:55 +0x328
github.com/k14s/ytt/pkg/yttlibrary/overlay.Op.Apply({{0x1469760, 0xc00000cb70}, {0x14f1640, 0xc0001a8cb0}, 0xc0001ea9b0, 0x0})
github.com/k14s/ytt/pkg/yttlibrary/overlay/op.go:27 +0x14f
github.com/k14s/ytt/pkg/workspace.OverlayPostProcessing.Apply({0xc000141a48})
github.com/k14s/ytt/pkg/workspace/overlay_post_processing.go:63 +0x716
github.com/k14s/ytt/pkg/workspace.(*LibraryExecution).Eval(0xc000141a48, 0x0, {0x0, 0x0, 0x1659478}, {0x0, 0xc00007d960, 0x0})
github.com/k14s/ytt/pkg/workspace/library_execution.go:191 +0x7b
github.com/k14s/ytt/pkg/cmd/template.(*Options).RunWithFiles(0xc00015e580, {{0xc00000e108, 0x2, 0x100c000141ae0}}, {0x165e748, 0xc0001a3380})
github.com/k14s/ytt/pkg/cmd/template/cmd.go:158 +0x567
github.com/k14s/ytt/pkg/cmd/template.(*Options).Run(0xc00015e580)
github.com/k14s/ytt/pkg/cmd/template/cmd.go:97 +0x4b1
github.com/k14s/ytt/pkg/cmd/template.NewCmd.func1(0x1359b85, {0xc00007e9a0, 0x0, 0x0})
github.com/k14s/ytt/pkg/cmd/template/cmd.go:62 +0x1d
github.com/cppforlife/cobrautil.WrapRunEForCmd.func1.1(0xc00017c600, {0xc00007e9a0, 0x0, 0x2})
github.com/cppforlife/cobrautil@v0.0.0-20200514214827-bb86e6965d72/misc.go:45 +0x6d
github.com/cppforlife/cobrautil.WrapRunEForCmd.func1.1(0xc000190a00, {0xc00007e9a0, 0x0, 0x2})
github.com/cppforlife/cobrautil@v0.0.0-20200514214827-bb86e6965d72/misc.go:45 +0x6d
github.com/spf13/cobra.(*Command).execute(0xc000190a00, {0xc00001e1c0, 0x2, 0x2})
github.com/spf13/cobra@v1.2.1/command.go:856 +0x60e
github.com/spf13/cobra.(*Command).ExecuteC(0xc000190a00)
github.com/spf13/cobra@v1.2.1/command.go:974 +0x3bc
github.com/spf13/cobra.(*Command).Execute(...)
github.com/spf13/cobra@v1.2.1/command.go:902
main.main()
github.com/k14s/ytt/cmd/ytt/ytt.go:21 +0xd6
```

**What did you expect:**
The document to be appended to the empty DocumentSet.

**Anything else you would like to add:**

Slack thread: https://kubernetes.slack.com/archives/CH8KCCKA5/p1637354198458400?thread_ts=1637349049.448900&cid=CH8KCCKA5

**Environment:**

- ytt version (use `ytt --version`): v0.38.0

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