Make LuckyPennySoftwareLicenseKey nullable

Updated the `LuckyPennySoftwareLicenseKey` property in the
`ConfigurationOptions` class to allow null values by changing
it from a non-nullable string with a default value of an empty
string to a nullable string (`string?`). This change enables
explicit representation of the absence of a license key using
`null` instead of an empty string.

src/ReC.Application/DependencyInjection.cs

@@ -27,6 +27,6 @@ public static class DependencyInjection
 
     public class ConfigurationOptions
     {
-        public string LuckyPennySoftwareLicenseKey { get; set; } = string.Empty;
+        public string? LuckyPennySoftwareLicenseKey { get; set; }
     }
 }