boobank: avoid blank lines between entries in (pretty_)qif formatting

rationale: empty lines between entries do not seem to be supported by
the spec
http://web.archive.org/web/20100222214101/http://web.intuit.com/support/quicken/docs/d_qif.html
and I have found at least one application reading QIF that works properly
without the blank lines, and fail to work with the blank lines

also: most QIF files generated by other apps in the wild seem not to have
any empty lines between entries