internal/http3: handle a nil TLSConfig in newQUICConfig

newQUICConfig read tlsConfig.NextProtos before checking tlsConfig for nil, so
a nil tls.Config panicked. The nil check that followed, after tlsConfig.Clone(),
was unreachable.

This is reachable from initEndpoint, which passes http.Transport.TLSClientConfig
straight through, and that field is nil for a default Transport:

    panic: runtime error: invalid memory address or nil pointer dereference
        golang.org/x/net/internal/http3.newQUICConfig(...)
        golang.org/x/net/internal/http3.(*transport).initEndpoint(...)

Change-Id: I8b8dcb56a2ffc148a03894335f9d74196a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/823105
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>