[Content Filtering] Crash when allowing a 0-byte resource to load

[Content Filtering] Crash when allowing a 0-byte resource to load
https://bugs.webkit.org/show_bug.cgi?id=150644
<rdar://problem/23288538>

Reviewed by Darin Adler.

Source/WebCore:

Test: contentfiltering/allow-empty-document.html

* loader/ContentFilter.cpp:
(WebCore::ContentFilter::deliverResourceData): resourceBuffer will be null if the resource contained no data.

LayoutTests:

* contentfiltering/allow-empty-document-expected.html: Added.
* contentfiltering/allow-empty-document.html: Added.
* contentfiltering/resources/empty.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@191706 268f45cc-cd09-0410-ab3c-d52691b4dbfc