🐛Linker: check for empty param values

[calebcordry]

Currently if a pub/vendor is using a config that looks something like this:

linkers: {
  testLinker1: {
    ids: {
      cid: 'QUERY_PARAM(cid)',
    }
  }
}

It is possible that the QUERY_PARAM macro resolves to an empty string, and therefore the linker value will resolve to an empty string. In this case we send a param like testLinker1=. This changes this behavior so that if a param value is empty it will not be appended.

[torch2424]

LGTM 😄 🚢

[zhouyx]

I understand you want the logic goes to createLinker and I agree this looks good now.
But depending on if we want to support wildcard domain matching, I think it may worth moving the check before the domain match check.

[calebcordry]

Sounds good. I will keep this in mind when we add this feature.