Use partial index on postgresql

only postgres supports them

server/src/main/resources/org/cloudfoundry/identity/uaa/db/postgresql/V4_110__Add_ExternalKey_To_Identity_Provider.sql

@@ -1,3 +1,3 @@
 -- add column external_key for oauth2,oidc,saml2 IdPs
 ALTER TABLE identity_provider ADD COLUMN external_key TEXT DEFAULT NULL;
-CREATE UNIQUE INDEX identity_provider_ext_key_zid__idx on identity_provider (identity_zone_id,type,external_key);
+CREATE UNIQUE INDEX identity_provider_ext_key_zid__idx on identity_provider (identity_zone_id,type,external_key) WHERE external_key IS NOT NULL;