fix of year in Birthdate

tgbotapi.core/src/commonMain/kotlin/dev/inmo/tgbotapi/types/Birthdate.kt

@@ -1,6 +1,8 @@
 package dev.inmo.tgbotapi.types
 
 import korlibs.time.Date
+import korlibs.time.DateTime
+import korlibs.time.Year
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
 
@@ -11,9 +13,12 @@ data class Birthdate(
     @SerialName(monthField)
     val month: Int,
     @SerialName(yearField)
-    val year: Int
+    val year: Int? = null
 ) {
+    /**
+     * Represents this birthday as korlibs [Date]. Will use this year in case if [year] has not been retrieved
+     */
     val date: Date by lazy {
-        Date(year, month, day)
+        Date(year ?: DateTime.now().year.year, month, day)
     }
 }