DevTools: [JsDoc] Mark Array types as strictly nullable or non-nullable

The introduction of this distinction has uncovered a few real bugs in our code,
fixed in this patch.

R=aandrey, pfeldman, vsevik
BUG=324831

Review URL: https://codereview.chromium.org/99513002

git-svn-id: svn://svn.chromium.org/blink/trunk@163093 bbb929c8-8fbe-4397-9dbb-9b2b20218538