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

include/core/SkWriter32.h

Index: include/core/SkWriter32.h
diff --git a/include/core/SkWriter32.h b/include/core/SkWriter32.h
index 82cf346c8e6cb9f1fed856b71fbd094a29e3cd08..308e28897fa860df98cb473ad4c9bab9d391e54f 100644
--- a/include/core/SkWriter32.h
+++ b/include/core/SkWriter32.h
@@ -168,7 +168,9 @@ public:
      *  Writes a string to the writer, which can be retrieved with
      *  SkReader32::readString().
      *  The length can be specified, or if -1 is passed, it will be computed by
-     *  calling strlen(). The length must be < 0xFFFF
+     *  calling strlen(). The length must be < max size_t.
+     *
+     *  If you write NULL, it will be read as "".
      */
     void writeString(const char* str, size_t len = (size_t)-1);