Secondary nav on mobile should only appear when there are nav items to show

[mmmavis]

Describe the bug

On mobile, secondary nav dropdown box is always there even when there's no nav item to show (the empty dropdown gets collapsed into a thick grey line).

To Reproduce

Steps to reproduce the behavior:

1. On mobile screen size, go to any primary / opportunity / campaign page that doesn't have child pages
2. Notice the thick grey line around header section.

Expected behavior

Empty dropdown / grey line shouldn't be there.

Screenshots

From https://foundation.mozilla.org/en/internet-health/

From https://foundation.mozilla.org/en/campaigns/equifax-breach/

/cc @beccaklam @kristinashu