Fix map path when HttpContext.Current is null
Fix map path when HttpContext.Current is null

Sometimes Nexu is initializing while HttpContext.Current is null.
This has the consequence that the grid config becomes empty,
because it maps to an non-existing file, and all grid editors fail
to work in Umbraco Backoffice requiring the site to be restarted.

dawoe/umbraco-nexu,dawoe/umbraco-nexu,dawoe/umbraco-nexu