patman: Issue empty change logs for unchanged patches

Often a particular patch may change only for some versions of a series.
For versions where there is no change, issue a change log indicating
that (for example 'Changes in v4: None').

For such lines, don't add a blank line afterwards, to conserve space.
Use list.insert() instead of list = [item] + list.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Stefan Roese <sr@denx.de>