fix for pc apply: add site, change entry type, localise relations

[i-just]

Description

Replication instructions: #12702 (comment)

What’s happening:

• When applying external project config changes, BaseRelationField->afterSave() adds the LocalizeRelations job to the queue
• but then the change for an entryType (e.g. from hasTitleField: true to false) triggers Sections->handleChangedEntryType()
• which, for singles, triggers Sections->_ensureSingleEntry(), which triggers the element to be saved and propagated
• that then triggers Relations->saveRelations(), which saves the existing relation that had sourceSiteId of null to sourceSiteId of the newly added site
• when the LocalizeRelations job is ready to do its job, there are no relations to localise cause the one it was supposed to localise now has the sourceSiteId of the newly added site
• this results in the relation being present in the newly added site but missing from the primary one

The fix:
Only attempt to save relations if we’re not applying PC changes to a Single.

Related issues

#12702