URLs created by mailsender plugin called from a scheduler script can have a wrong context path

This happens when having XWiki as the ROOT webapp.

The scenario is the following :

• Scheduler job calls mailsender.sendMailFromTemplate
• mail sender creates a new ExternalServletURLFactory, initialized with the calling context, which has a XWikiServletRequestStub as request
• URL factory initialization calls XWiki#getWebAppPath to get the context path
• #getWebAppPath calls contextPath = context.getRequest().getContextPath(); which generates a NPE
• ... NPE is caught and a wrong context path is computed from the context URL, base on context.getURL().getPath();

For example, <host>/bin/bin/Sandbox/Test, when the servlet path is the standard one.