net/url: consistently remove ../ elements in JoinPath

net/url: consistently remove ../ elements in JoinPath

JoinPath would fail to remove relative elements from the start of
the path when the first path element is "".

In addition, JoinPath would return the original path unmodified
when provided with no elements to join, violating the documented
behavior of always cleaning the resulting path.

Correct both these cases.

    JoinPath("http://go.dev", "../go")
    // before: http://go.dev/../go
    // after:  http://go.dev/go

    JoinPath("http://go.dev/../go")
    // before: http://go.dev/../go
    // after:  http://go.dev/go

Fixes #54385.

Change-Id: I6d22cd160d097c50703dd96e4f453c6c118fd5d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/423514
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>