Nullpointer Exception in Token.fromString()

[sdoward]

The method Token.fromString(@Nullable String) throws a NullPointer when passing a null String into the method. This is missleading for a couple of reasons

1. The method is annotated with @Nullable so it would seem it acceptable to pass a null object into it. It seems that this annotation should be @NotNull
2. The stack trace lead me to looking into the internals of JSONTokener which is were the error originates from. Rather than giving a reasonable stacktrace and message so developers can diagnose the issue. A exception could be thrown with a message such "A null string cannot be converted to a token" if a null string is passed into the method.

java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.length()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2673)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2734)
        at android.app.ActivityThread.-wrap12(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1485)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6195)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:889)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:779)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.length()' on a null object reference
        at org.json.JSONTokener.nextCleanInternal(JSONTokener.java:116)
        at org.json.JSONTokener.nextValue(JSONTokener.java:94)
        at org.json.JSONObject.<init>(JSONObject.java:156)
        at org.json.JSONObject.<init>(JSONObject.java:173)
        at com.stripe.android.model.Token.fromString(Token.java:160) ```

[ksun-stripe]

Hi @sdoward! Thanks so much for bringing this our attention :) Sorry about your time spend debugging. I've put a PR to handle null there the same way the other methods in the class handle null.

#642