eslint-plugin-react-hooks version: 4.2.0

## Steps To Reproduce

I know it shouldn't exist in production code, but it's annoying while editing some files.

```js
import React, { useEffect } from "react";

const App = () => {
  useEffect(); // <-
  return null;
};

export default App;
```

Link to code example: https://codesandbox.io/s/polished-sun-3wr27?file=/src/App.js

## The current behavior

ESLint crashed.

```
TypeError: Cannot read property 'type' of undefined
```

## The expected behavior

ESLint doesn't crash but reports an error instead.
if (deps.size === 0) {
          return null;
        }
        return (
          (deps.size > 1 ? '' : singlePrefix + ' ') +
          label +
          ' ' +
          (deps.size > 1 ? 'dependencies' : 'dependency') +
          ': ' +
          joinEnglish(
            Array.from(deps)
              .sort()
              .map(name => "'" + formatDependency(name) + "'"),
          ) +
          `. Either ${fixVerb} ${
            deps.size > 1 ? 'them' : 'it'
          } or remove the dependency array.`
        );
      }
      let extraWarning = '';
      if (unnecessaryDependencies.size > 0) {
        let badRef = null;
        Array.from(unnecessaryDependencies.keys()).forEach(key => {
          if (badRef !== null) {
            return;
          }