Empty hst:cmspreviewprefix property gives double slashes in the URL of a previewed channel with a mount path

When the property 'hst:cmspreviewprefix' is empty (preferred in production environments where the CMS host differs from the site host), the URLs of a previewed channel from the channel manager contain a double '/'. The URL still works and the previewed site is shown correctly, but it looks a bit sloppy.

Steps to reproduce on the test server:
1. go to http://cms.demo.test.onehippo.com/
2. log in as admin
3. navigate to the channel overview
4. click on the 'preview' link of the 'Dutch Website' channel

Expected behavior: the URL of the previewed channel is http://cms.demo.test.onehippo.com/site/nl
Actual behavior: the URL of the previewed channel is http://cms.demo.test.onehippo.com/site//nl (notice the double slash before 'nl' at the end)

Steps to reproduce locally:
1. set up HTTP with a different host for the CMS and the site
2. create a HST host group the local site behind httpd:

Copy the dev-localhost group to a new node 'dev-external'

/hst:hst/hst:hosts

• hst:channelmanagerhostgroup=dev-external

/hst:hst/hst:hosts/dev-external

• hst:cmspreviewprefix=<empty string>
• hst:cmslocation=<local cms url>

/hst:hst/hst:hosts/dev-external/<root virtualhost node>

• hst:showcontentpath=false
• hst:showport=false

3. go to http:// <local cms url>
4. log in as admin
5. navigate to the channel overview
6. click on the 'preview' link of the 'Dutch Website' channel