Fix URI.merge leaking :+ marker when base path is empty string

[pnezis]

When merging URIs where the base has path: "", the internal :+ marker used to track the join point between base and relative paths was being converted to a literal "+" in the output.

iex(1)> base = %URI{scheme: "http", host: "example.com", path: ""}
iex(2)> URI.merge(base, "foo") |> to_string()
"http://example.com/+/foo"

[josevalim]

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