library: keep the cached bio on an empty refresh

app/src/main/kotlin/com/subtracks/data/repo/LibraryRepository.kt

@@ -367,8 +367,11 @@ class LibraryRepository(
         artistId: String,
     ): Result<Unit> =
         try {
-            val biography = normalizeBiography(sourceRepository.artistBiography(sourceId, artistId))
+            val fetched = normalizeBiography(sourceRepository.artistBiography(sourceId, artistId))
             val current = db.libraryDao().artistBioOnce(sourceId, artistId)
+            // An empty response can mean the metadata provider is temporarily down, so keep the
+            // cached biography instead of erasing it.
+            val biography = fetched ?: current?.biography
             if (current == null || current.biography != biography) {
                 db.libraryDao().upsertArtistBio(ArtistBio(sourceId, artistId, biography))
             }

app/src/test/kotlin/com/subtracks/data/repo/LibraryRepositoryTest.kt

@@ -311,6 +311,24 @@ class LibraryRepositoryTest {
             }
         }
 
+    @Test
+    fun anEmptyRefreshKeepsTheStoredBio() =
+        runBlocking {
+            withSource { server ->
+                val sourceId = sourceRepository.activeSourceIdOnce()!!
+                db.libraryDao().upsertArtistBio(ArtistBio(sourceId, "ar1", "Kept"))
+                server.enqueue(
+                    MockResponse().setBody(
+                        "<subsonic-response status=\"ok\"><artistInfo2/></subsonic-response>",
+                    ),
+                )
+
+                assertTrue(repository.refreshArtistBio(sourceId, "ar1").isSuccess)
+
+                assertEquals("Kept", repository.artistBio(sourceId, "ar1").first()?.biography)
+            }
+        }
+
     @Test
     fun aFailedRefreshKeepsTheStoredBio() =
         runBlocking {