Don't store blank usernames.

app/src/main/java/org/thoughtcrime/securesms/keyvalue/AccountValues.kt

@@ -362,7 +362,14 @@ internal class AccountValues internal constructor(store: KeyValueStore) : Signal
     get() = !isPrimaryDevice
 
   /** The local user's full username (nickname.discriminator), if set. */
-  var username: String? by stringValue(KEY_USERNAME, null)
+  var username: String?
+    get() {
+      val value = getString(KEY_USERNAME, null)
+      return if (value.isNullOrBlank()) null else value
+    }
+    set(value) {
+      putString(KEY_USERNAME, value)
+    }
 
   /** The local user's username link components, if set. */
   var usernameLink: UsernameLinkComponents?

app/src/main/java/org/thoughtcrime/securesms/migrations/CopyUsernameToSignalStoreMigrationJob.kt

@@ -32,7 +32,7 @@ internal class CopyUsernameToSignalStoreMigrationJob(
 
     val self = Recipient.self()
 
-    if (self.username.isEmpty) {
+    if (self.username.isEmpty || self.username.get().isBlank()) {
       Log.i(TAG, "No username set, skipping.")
       return
     }