fix: enhance FieldValue with null handling and type-safe access (#1)

- Implement NullValueHandling.Ignore for JSON serialization
- Change Value property from dynamic to object for clarity
- Add GetValue<T>() method for type-safe value retrieval

QuickbaseNet/Models/FieldValue.cs

@@ -1,10 +1,16 @@
 using Newtonsoft.Json;
+using System;
 
 namespace QuickbaseNet.Models
 {
     public class FieldValue
     {
-        [JsonProperty("value")]
-        public dynamic Value { get; set; }
+        [JsonProperty("value", NullValueHandling = NullValueHandling.Ignore)]
+        public object Value { get; set; }
+
+        public T GetValue<T>()
+        {
+            return Value == null ? default : (T)Convert.ChangeType(Value, typeof(T));
+        }
     }
 }