User table: ensure bio is always a string

Gets rid of '|| ""' in multiple places and fixes #2067

test/user_test.exs

@@ -1466,7 +1466,7 @@ defmodule Pleroma.UserTest do
     user = User.get_by_id(user.id)
 
     assert %User{
-             bio: nil,
+             bio: "",
              raw_bio: nil,
              email: nil,
              name: nil,

test/web/admin_api/controllers/admin_api_controller_test.exs

@@ -203,7 +203,7 @@ defmodule Pleroma.Web.AdminAPI.AdminAPIControllerTest do
         assert user.note_count == 0
         assert user.follower_count == 0
         assert user.following_count == 0
-        assert user.bio == nil
+        assert user.bio == ""
         assert user.name == nil
 
         assert called(Pleroma.Web.Federator.publish(:_))

test/web/twitter_api/util_controller_test.exs

@@ -594,7 +594,7 @@ defmodule Pleroma.Web.TwitterAPI.UtilControllerTest do
       user = User.get_by_id(user.id)
       assert user.deactivated == true
       assert user.name == nil
-      assert user.bio == nil
+      assert user.bio == ""
       assert user.password_hash == nil
     end
   end