Fix Access behavior error in DeviceParser

Replace bracket notation (packet_data[key]) with Map.get/2 to avoid
UndefinedFunctionError when processing ParseError structs that don't
implement the Access behavior.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

lib/aprsme/device_parser.ex

@@ -23,8 +23,8 @@ defmodule Aprsme.DeviceParser do
   end
 
   defp get_field_value(packet_data, key) do
-    if Map.has_key?(packet_data, key) and not is_nil(packet_data[key]) do
+    if Map.has_key?(packet_data, key) do
-      packet_data[key]
+      Map.get(packet_data, key)
     end
   end