Created attachment 122921 [details] bottom right show the case of a marker on an empty line In regard to markers there is no difference between line, path and polyline. Therefore a marker should be rendered on an empty line the same way as it is rendered on an empty polyline or an empty path.
In https://www.w3.org/TR/SVG/implnote.html#PathElementImplementationNotes you find "If markers are specified, then a marker is drawn on every applicable vertex, even if the given vertex is the end point of a zero-length path segment and even if "moveto" commands follow each other." Inkscape, Seamonkey, Chrome and Batik show a marker for a zero-length line. LibreOffice should do that too.
Regina Henschel committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/core/commit/?id=74040d447912eff5f7366b8ae61244ad101000dc tdf#98119 Allow markers on zero-length lines It will be available in 5.2.0. The patch should be included in the daily builds available at http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More information about daily builds can be found at: http://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.