User table: ensure bio is always a string

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

priv/repo/migrations/20200901061256_ensure_bio_is_string.exs

@@ -0,0 +1,7 @@
+defmodule Pleroma.Repo.Migrations.EnsureBioIsString do
+  use Ecto.Migration
+
+  def change do
+    execute("update users set bio = '' where bio is null", "")
+  end
+end

priv/repo/migrations/20200901061637_bio_set_not_null.exs

@@ -0,0 +1,10 @@
+defmodule Pleroma.Repo.Migrations.BioSetNotNull do
+  use Ecto.Migration
+
+  def change do
+    execute(
+      "alter table users alter column bio set not null",
+      "alter table users alter column bio drop not null"
+    )
+  end
+end