vars: Make nil values act as empty string instead of "<nil>"

[francislavoie]

This is a silly little bug.

I was looking into a possible config to fix an issue for Authelia, and while testing I noticed that vars and vars_regexp matchers were turning placeholders that return no value (nil) into literally the string "<nil>" because of the default: case doing fmt.Sprintf("%v", vv).

This means that it was impossible to match against nil placeholders by checking for empty string, because the value would always be non-empty. So this fixes a matchers like this:

@foo not vars {rp.header.Foo} ""
@bar vars_regexp bar {rp.header.Bar} ^(.+)$

These matchers when inside a proxy handle_response should not match when the response had a non-empty header value (first one because of not negating empty string, and the regexp requiring at least one character), but instead it did match because the actual string was <nil>.

[mholt]

I suppose this is better... thanks. I'm not 100% convinced it will be for everyone, though, as in some cases I could see a nil value wanting to be explicitly printed as nil, as opposed to empty, but, let's see how it goes 👍