Return null instead of throwing when User Object attribute helper is …

…called with invalid value

src/User.ts

@@ -2,46 +2,48 @@ import { isStringArray } from "./RolloutEvaluator";
 
 export type WellKnownUserObjectAttribute = "Identifier" | "Email" | "Country";
 
+type AttributeValue<T, U> = T extends U ? string : null;
+
 /** User Object. Contains user attributes which are used for evaluating targeting rules and percentage options. */
 export class User {
   /**
    * Converts the specified `Date` value to the format expected by datetime comparison operators (BEFORE/AFTER).
    * @param date The `Date` value to convert.
    * @returns The User Object attribute value in the expected format.
   */
-  static attributeValueFromDate(date: Date): string {
+  static attributeValueFromDate<T>(date: T): AttributeValue<T, Date> {
     if (!(date instanceof Date)) {
-      throw new Error("Invalid 'date' value");
+      return null as AttributeValue<T, Date>;
     }
 
     const unixTimeSeconds = date.getTime() / 1000;
-    return unixTimeSeconds + "";
+    return (unixTimeSeconds + "") as AttributeValue<T, Date>;
   }
 
   /**
    * Converts the specified `number` value to the format expected by number comparison operators.
    * @param number The `number` value to convert.
    * @returns The User Object attribute value in the expected format.
   */
-  static attributeValueFromNumber(number: number): string {
+  static attributeValueFromNumber<T>(number: T): AttributeValue<T, number> {
     if (typeof number !== "number" || !isFinite(number)) {
-      throw new Error("Invalid 'date' value");
+      return null as AttributeValue<T, number>;
     }
 
-    return number + "";
+    return (number + "") as (AttributeValue<T, number>);
   }
 
   /**
    * Converts the specified `string` items to the format expected by array comparison operators (ARRAY CONTAINS ANY OF/ARRAY NOT CONTAINS ANY OF).
    * @param items The `string` items to convert.
    * @returns The User Object attribute value in the expected format.
   */
-  static attributeValueFromStringArray(...items: ReadonlyArray<string>): string {
+  static attributeValueFromStringArray<T>(...items: ReadonlyArray<T>): AttributeValue<T, string> {
     if (!isStringArray(items)) {
-      throw new Error("Invalid 'items' value");
+      return null as AttributeValue<T, string>;
     }
 
-    return JSON.stringify(items);
+    return JSON.stringify(items) as AttributeValue<T, string>;
   }
 
   constructor(