For example, when filing Form 1120 for 2018 by mail, if I've left all entry spaces on page 2 (Schedule C) blank, may I leave out page 2?
When I look at the instructions for Form 1120 there is a section titled 'Assembling the Return'. I do not see any instructions saying any of the pages or schedules are optional.
This is a case of 'better safe than sorry'. I would not invite further scrutiny on my tax return just to save the marginal cost of printing and mailing one sheet of paper.
A blank field in a mailed form is different than a page that wasn't sent. You could later argue that you did send the page with the fields filled out. The IRS will not accept a form with pages missing.
with that said I don't see how you can leave them blank. It seems like there should be a 0 in all of the fields.