fix(aprs): handle empty APRS_DATABASE_URL and dedupe boot warning

- Empty-string env var (k8s secret resolves to '') was passing the nil
  check and handing Postgrex an empty URL — now treated the same as unset.
- Drop runtime.exs warning; keep the single warning in application.ex
  where the supervisor decision is made.

config/runtime.exs

@@ -359,20 +359,18 @@ if config_env() == :prod do
 
   # Optional secondary repo for read-only access to aprs.me's database.
   # Used by APRS-based 144 MHz calibration tooling. When the env var is
-  # unset we leave the AprsRepo unconfigured; Microwaveprop.Application
-  # logs a warning and skips starting it, and the rest of the app boots
-  # normally.
+  # unset OR an empty string (k8s secret resolves to "") we leave the
+  # AprsRepo unconfigured; Microwaveprop.Application is the single site
+  # that logs the skip and decides not to start it.
   case System.get_env("APRS_DATABASE_URL") do
-    nil ->
-      Logger.warning("APRS_DATABASE_URL not set — Microwaveprop.AprsRepo will not start; APRS calibration disabled.")
-
-      :ok
-
-    aprs_url ->
+    url when is_binary(url) and url != "" ->
       config :microwaveprop, Microwaveprop.AprsRepo,
-        url: aprs_url,
+        url: url,
         pool_size: String.to_integer(System.get_env("APRS_POOL_SIZE") || "2"),
         socket_options: maybe_ipv6
+
+    _ ->
+      :ok
   end
 
   config :microwaveprop, MicrowavepropWeb.Endpoint,