Summary:
In the wild, our app will occasionally crash with:
```
Fatal Exception: java.lang.NullPointerException
value == null
```
The stack trace brings it back to `okhttp3.Headers$Builder.checkNameAndValue (Headers.java:316)`:
```
if (value == null) throw new NullPointerException("value == null");
```
In the proposed fix, we simply continue the documented functionality of the `extractHeaders` method by returning "null" for invalid data.
Closes https://github.com/facebook/react-native/pull/10861
Differential Revision: D4178624
Pulled By: ericvicenti
fbshipit-source-id: 632e742196339639cb57ea47f9d0efbf04f090be
