[bug] Fix NullPointerException when error stream does not contain error json field

[damienbeaufils]

Hello Prismic team,

We've noticed a NullPointerException error which can occur when Prismic API returns an error with a body which does not contain the error json field.
The corresponding stracktrace:

java.lang.NullPointerException: null
    at io.prismic.core.HttpClient.fetch(HttpClient.java:62)
    at io.prismic.Form$SearchForm.submit(Form.java:411)

This PR handle this NPE and fix a test which fails since March 12th because of the default CDN activation.

Thanks!

[arnaudlewis]

Thanks for your contribution @damienbeaufils.
We need to identify the source before merging, I'll keep you posted.

[bobsoppe]

@arnaudlewis any update on this PR?