fix ContentTag#direct_shareable? for nil content_id
fix ContentTag#direct_shareable? for nil content_id

in practice I can't find any module tags fitting this description
but just in case that happens

test plan: specs

refs LS-1254

Change-Id: Iadaa9915bce20b96fb4b195df99f97fe7dd09d56
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/244821
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: James Williams <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
QA-Review: James Williams <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
Product-Review: James Williams <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms