fixes in new "asInputMedia" functions

src/commonMain/kotlin/com/github/insanusmokrassar/TelegramBotAPI/types/message/content/media/AnimationContent.kt

@@ -45,7 +45,7 @@ data class AnimationContent(
 
     override fun asInputMedia(): InputMediaAnimation = InputMediaAnimation(
         media.fileId,
-        toMarkdownV2Captions().first(),
+        toMarkdownV2Captions().firstOrNull(),
         MarkdownV2,
         media.width,
         media.height,

src/commonMain/kotlin/com/github/insanusmokrassar/TelegramBotAPI/types/message/content/media/AudioContent.kt

@@ -44,7 +44,7 @@ data class AudioContent(
 
     override fun asInputMedia(): InputMediaAudio = InputMediaAudio(
         media.fileId,
-        toMarkdownV2Captions().first(),
+        toMarkdownV2Captions().firstOrNull(),
         MarkdownV2,
         media.duration,
         media.performer,

src/commonMain/kotlin/com/github/insanusmokrassar/TelegramBotAPI/types/message/content/media/DocumentContent.kt

@@ -41,7 +41,7 @@ data class DocumentContent(
 
     override fun asInputMedia(): InputMediaDocument = InputMediaDocument(
         media.fileId,
-        toMarkdownV2Captions().first(),
+        toMarkdownV2Captions().firstOrNull(),
         MarkdownV2,
         media.thumb ?.fileId
     )

src/commonMain/kotlin/com/github/insanusmokrassar/TelegramBotAPI/types/message/content/media/PhotoContent.kt

@@ -48,7 +48,7 @@ data class PhotoContent(
 
     override fun asInputMedia(): InputMediaPhoto = InputMediaPhoto(
         media.fileId,
-        toMarkdownV2Captions().first(),
+        toMarkdownV2Captions().firstOrNull(),
         MarkdownV2
     )
 }

src/commonMain/kotlin/com/github/insanusmokrassar/TelegramBotAPI/types/message/content/media/VideoContent.kt

@@ -53,7 +53,7 @@ data class VideoContent(
 
     override fun asInputMedia(): InputMediaVideo = InputMediaVideo(
         media.fileId,
-        toMarkdownV2Captions().first(),
+        toMarkdownV2Captions().firstOrNull(),
         MarkdownV2,
         media.width,
         media.height,

src/commonMain/kotlin/com/github/insanusmokrassar/TelegramBotAPI/types/message/content/media/VoiceContent.kt

@@ -42,7 +42,7 @@ data class VoiceContent(
 
     override fun asInputMedia(): InputMediaAudio = InputMediaAudio(
         media.fileId,
-        toMarkdownV2Captions().first(),
+        toMarkdownV2Captions().firstOrNull(),
         MarkdownV2,
         media.duration
     )