ScopedStyleResolver::hasOnlyEmptyRuleSets should consider the case: !m_authorStyle.

When checking whether it is possible to destroy ScopedStyleResolver, its hasOnlyEmptyRuleSets method is invoked.
However, if the ScopedStyleResolver has no rules, its m_authorStyle might be null.
So "m_authorStyle->ruleCount()" causes crash reported in crbug.com/305817.

BUG=305817
TEST=fast/dom/shadow/style-insertion-crash.html

Review URL: https://codereview.chromium.org/26420004

git-svn-id: svn://svn.chromium.org/blink/trunk@159345 bbb929c8-8fbe-4397-9dbb-9b2b20218538