webview: handle null in pinning impl.

The code to pin WebView wasn't handling sharedLibraryFiles being null;
the Google-branded WebView always has at least one shared library
dependency, but the AOSP build isn't guaranteed to and was causing an
exception to be thrown here in some configurations, skipping both the
pinning and also later parts of WebView initialization.

Bug: 307594624
Bug: 331842293
Test: boot using older AOSP WebView, check logcat and dumpsys pinner
Change-Id: I6b078e603d7cdcad6b443093789295d85835c574