Add AM/PM marker if 12-hour time format does not contain it

Fixes #23

lib/system_date_time_format.dart

@@ -60,7 +60,16 @@ class SystemDateTimeFormat {
   Future<String?> getTimePattern() async {
     final pattern =
         await SystemDateTimeFormatPlatformInterface.instance.getTimePattern();
-    return pattern.isNullOrEmpty ? null : pattern;
+
+    if (pattern.isNullOrEmpty) return null;
+
+    // Add an AM/PM marker if the 12-hour time pattern does not contain it.
+    // Fixes https://github.com/Nikoro/system_date_time_format/issues/23
+    if (pattern == 'h:mm' || pattern == 'hh:mm') {
+      return '$pattern a';
+    }
+
+    return pattern;
   }
 
   /// Returns all available date & time patterns.