Add not null constraint on external_diff_store

But ignoring existing rows.

After we do this, then we update existing rows. Then we can enforce no
`NULL`s.

db/post_migrate/20200804041930_add_not_null_constraint_on_external_diff_store_to_merge_request_diffs.rb

+# frozen_string_literal: true
+
+class AddNotNullConstraintOnExternalDiffStoreToMergeRequestDiffs < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_not_null_constraint(:merge_request_diffs, :external_diff_store, validate: false)
+  end
+
+  def down
+    remove_not_null_constraint(:merge_request_diffs, :external_diff_store)
+  end
+end

db/schema_migrations/20200804041930

+3177e2f9549add35789e64b14c5958c4fb42fdce4b4705505bc9bbe326d9a875
\ No newline at end of file

db/structure.sql

@@ -17613,6 +17613,9 @@ ALTER TABLE public.design_management_designs
 ALTER TABLE public.vulnerability_scanners
     ADD CONSTRAINT check_37608c9db5 CHECK ((char_length(vendor) <= 255)) NOT VALID;
 
+ALTER TABLE public.merge_request_diffs
+    ADD CONSTRAINT check_93ee616ac9 CHECK ((external_diff_store IS NOT NULL)) NOT VALID;
+
 ALTER TABLE ONLY public.ci_build_needs
     ADD CONSTRAINT ci_build_needs_pkey PRIMARY KEY (id);