Continuous (single-digit) versioning

**Describe the bug**

`v0` + `major` bump = `null` 💥

**To Reproduce**

- latest (and the only) git (annotated) tag is `v0`
- default release type is `major` to always increment the first digit
- running `auto latest --dry-run`:

```
✔ Latest Release: v0 (...​)
...
✔ success Calculated SEMVER bump: major
ℹ info Would have created a release on GitHub for version: null
```

**Expected behavior**

`v0` + `major` bump = `v1`

(or generally `vN` + `major` bump = `v(N+1)`)

**Environment information:**

Redacted (private repo):

```txt
Environment Information:

"auto" version: v10.20.0
"git" version: v2.30.2
"node" version: v12.18.1

Project Information:

...
✔ Current Version: v0
✔ Latest Release: v0 (​https://github.com/.../releases/tag/v0​)

✔ Labels configured on GitHub project
```

Standalone Auto installation (non-npm project). Plugins:

```json
{
"plugins": [
[
"docker",
{
"registry": "...",
"image": "..."
}
],
"released"
],
"shipit": {
"noChangelog": true
},
"latest": {
"noChangelog": true
},
"canary": {
"force": true
},
"labels": [
{
"name": "release",
"changelogTitle": "🚀 Enhancement",
"description": "Any change, feature or enhancement",
"releaseType": "major",
"default": true
},
{
"name": "no-release",
"changelogTitle": "🏠 Internal",
"description": "Changes that don't touch user-facing code",
"releaseType": "none"
}
]
}
```

**Additional context**

I'm not sure if this behavior is meant to supported, but I'm filing it as a bug because `null` is never a valid answer 😄