Oats requires the server directive

A swagger definition is valid without this directive, but oats requires it.

Given the following swagger:
```yaml
openapi: "3.0.0"
info:
title: Stuff service
version: 0.0.0
paths:
/stuff:
post:
operationId: createStuff
requestBody:
description: thing
content:
application/json:
schema:
type: string
responses:
'204':
description: No content
```

Oats returns the following error:
```
/src/node_modules/@influxdata/oats/dist/generate.js:32
server: this.doc.servers[0].url,
^

TypeError: Cannot read property '0' of undefined
at Generator.registerPathOperation (/src/node_modules/@influxdata/oats/dist/generate.js:32:37)
at new Generator (/src/node_modules/@influxdata/oats/dist/generate.js:24:26)
at generate (/src/node_modules/@influxdata/oats/dist/generate.js:219:23)
at async Command. (/src/node_modules/@influxdata/oats/bin/oats:7:18)
```

Run with:
```sh
docker run -v ${PWD}:/test --entrypoint /src/node_modules/.bin/oats quay.io/influxdb/oats /test/swagger.yaml
```

Reproduce the failure with the provided Docker command and swagger definition, then inspect dist/generate.js, especially registerPathOperation, and the bin/oats entry point. Done means this valid OpenAPI definition runs without the TypeError when no servers directive is present.

Written by the indexing model from the issue text.