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.
);
              return; 
            case 'VariableDeclarator':
              const init = def.node.init;
              if (!init) {
                break; 
              }
              switch (init.type) {
                
                
                case 'ArrowFunctionExpression':
                case 'FunctionExpression':
                  
                  visitFunctionWithDependencies(
                    init,
                    declaredDependenciesNode,
                    reactiveHook,
                    reactiveHookName,
                    isEffect,
                  );
                  return; 
              }
              break; 
          }
          break; 
        default:
          
          reportProblem({
            node: reactiveHook,
            message: