Early exit when GlyphBuffer contains no FontData

This is a follow-up CL of r166096, which added gracefull handling of
missing glyph. We should also treat the case in fillGlyphBuffer().
No assertion added because the assert in
setGlyphPositionsForHarfBuzzRun() will catch the problem.

Since drawGlyphBuffer() assumes that glyphBuffer is not empty, we
should not call drawGlyphBuffer() when it is empty.

BUG=359664

Review URL: https://codereview.chromium.org/224983003

git-svn-id: svn://svn.chromium.org/blink/trunk@170921 bbb929c8-8fbe-4397-9dbb-9b2b20218538