(#649) Remove blank helthcheck user agents and paths when setting up the healthcheck middleware

A blank user agent is considered == to an empty string. When no -ping-user-agent option is specified, this is considered to be an empty string.

This reveals two problems:
- When no ping-user-agent is specified, main.go sets up a health check user agent of ""
- When no user agent is specified, the empty string is still checked against the health check user agents.

Now the health check middleware ignores blank user agents and paths in order to sanitise it's input to avoid this issue.

Additional tests have been added to verify these situations.