Cosmetic change to avoid dangling suffix of ':' if source is nil (#23)

As it stands, when source is nil (which, for example, can happen if there is a
call to `Repo.transaction`), the name of the span ends in an odd ':'. This
removes that ':'.

Co-authored-by: Bryan Naegele <bryannaegele@users.noreply.github.com>

instrumentation/opentelemetry_ecto/lib/opentelemetry_ecto.ex

@@ -57,7 +57,7 @@ defmodule OpentelemetryEcto do
       case Keyword.fetch(config, :span_prefix) do
         {:ok, prefix} -> prefix
         :error -> Enum.join(event, ".")
-      end <> ":#{source}"
+      end <> if source != nil, do: ":#{source}", else: ""
 
     time_unit = Keyword.get(config, :time_unit, :microsecond)