Revert "Avoid header line with some empty non-nil formats"

Revert "Avoid header line with some empty non-nil formats"

This reverts commit 4f66cbbfe520ee31ef26676e09a926217d9736fe.
This is part of removing the recently-added feature whereby
certain non-nil values of 'header-line-format' could signal
that the header line shall not be displayed.  The feature is
being reverted because its advantages are very minor, whereas
the complications it causes are serious.