fix: ExchangeVendor valueOf return null when not exists in list

reactive-crypto-core/src/main/kotlin/com/njkim/reactivecrypto/core/common/model/ExchangeVendor.kt

@@ -58,7 +58,7 @@ data class ExchangeVendor(val name: String) {
 
         @JvmStatic
         fun valueOf(value: String): ExchangeVendor {
-            return mapCache[value] ?: throw NoSuchElementException()
+            return mapCache[value] ?: ExchangeVendor(value)
         }
 
         @JvmStatic