Write NULL as "" so readString() always returns a non-NULL string.

BUG=
R=reed@google.com

Review URL: https://codereview.chromium.org/22862002

git-svn-id: http://skia.googlecode.com/svn/trunk@10754 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed