don't NPE if package_release_date isn't in a datablob

gson is happy to push nulls into non-nullable fields because we're
handing it a ::class.java. package_release_date sometimes doesn't exist
when release_date still does, so make package_release_date properly
nullable and fall back to release_date.

src/main/kotlin/bandcampcollectiondownloader/BandcampCollectionDownloader.kt

@@ -20,7 +20,8 @@ data class ParsedBandcampData(
 
 data class DigitalItem(
         val downloads: Map<String, Map<String, String>>,
-        val package_release_date: String,
+        val package_release_date: String?,
+        val release_date: String,
         val title: String,
         val artist: String,
         val download_type: String,
@@ -80,7 +81,7 @@ fun downloadAll(cookiesFile: Path?, bandcampUser: String, downloadFormat: String
         val digitalItem = downloadPageJsonParsed.digital_items[0]
         var albumtitle = digitalItem.title
         var artist = digitalItem.artist
-        val releaseDate = digitalItem.package_release_date
+        val releaseDate = digitalItem.package_release_date ?: digitalItem.release_date
         val releaseYear = releaseDate.subSequence(7, 11)
         val isSingleTrack: Boolean = digitalItem.download_type == "t"
         val url = digitalItem.downloads[downloadFormat]?.get("url").orEmpty()
@@ -207,4 +208,4 @@ fun prepareDownload(albumtitle: String, url: String, cookies: Map<String, String
 
     // Download content
     return downloadFile(realDownloadURL, albumFolderPath, timeout = timeout)
-}
+}