buildman: Ignore blank lines during size checks

Today when parsing the .sizes files we get a warning about an invalid
line in the file as it's blank.  Solve this by checking that we have a
non-blank line prior to processing.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>