fix(Marker): allow null and undefined as children
fix(Marker): allow null and undefined as children

* Original commit: f914c6f630f3beb34d96e1a053fef49d43bf736a
* Original author: @idolizesc
* Closes #119

This is useful is you have code like the following:

```js
<Marker {...props}>
  { showInfo &&
    <InfoWindow content="Some Info" />
  }
</Marker>
```

Right now, this will throw an `Error` about being unable to read `props` of `undefined`, so you have to use the following workaround (not ideal):

```js
// workaround:
<Marker {...props}>
  { showInfo ?
    <InfoWindow content="Some Info" />
    : <noscript />
  }
</Marker>
```

RishabhJain96/react-google-maps,jossi87/react-google-maps,joshcarr/react-google-maps,tomchentw/react-google-maps,ksavenkov/react-google-maps,jossi87/react-google-maps,RishabhJain96/react-google-maps