Avoids creating nip94 and nip95 events with blank alts

quartz/src/main/java/com/vitorpamplona/quartz/events/FileHeaderEvent.kt

@@ -62,7 +62,7 @@ class FileHeaderEvent(
             val tags = listOfNotNull(
                 listOf(URL, url),
                 mimeType?.let { listOf(MIME_TYPE, mimeType) },
-                alt?.let { listOf(ALT, it) },
+                alt?.ifBlank { null }?.let { listOf(ALT, it) },
                 hash?.let { listOf(HASH, it) },
                 size?.let { listOf(FILE_SIZE, it) },
                 dimensions?.let { listOf(DIMENSION, it) },

quartz/src/main/java/com/vitorpamplona/quartz/events/FileStorageHeaderEvent.kt

@@ -59,7 +59,7 @@ class FileStorageHeaderEvent(
             val tags = listOfNotNull(
                 listOf("e", storageEvent.id),
                 mimeType?.let { listOf(MIME_TYPE, mimeType) },
-                alt?.let { listOf(ALT, it) },
+                alt?.ifBlank { null }?.let { listOf(ALT, it) },
                 hash?.let { listOf(HASH, it) },
                 size?.let { listOf(FILE_SIZE, it) },
                 dimensions?.let { listOf(DIMENSION, it) },