fix: guard nil invitation from AddCollaborator for org members (#3371)

When the invitee of `github_repository_collaborators` is an organization
member who is not yet a direct collaborator, GitHub's REST API returns
`204 No Content` from `PUT /repos/{owner}/{repo}/collaborators/{username}`
(the user is promoted to direct collaborator immediately, no invitation is
issued). The Go client surfaces this as `inv == nil, err == nil`.

Since #3233 the create/update path in `updateUserCollaboratorsAndInvites`
unconditionally dereferences `inv.ID`, causing a nil pointer panic for
any configuration that adds an org member as a direct collaborator for
the first time.

Guard the invitation-tracking branch on `inv != nil`. On the 204 path
there is no invitation to record; the user is a direct collaborator
already and will be picked up by the next `listUserCollaborators` call.

Signed-off-by: bofus10 <33520078+bofus10@users.noreply.github.com>

Author: bofus10

github/resource_github_repository_collaborators.go

@@ -691,8 +691,13 @@ func updateUserCollaboratorsAndInvites(ctx context.Context, client *github.Clien
 		if err != nil {
 			return nil, err
 		}
-		inUser.invitationID = inv.ID
-		ghInvites = append(ghInvites, inUser)
+		// AddCollaborator returns 204 No Content (inv == nil) when the invitee
+		// is an organization member gaining direct access without an
+		// invitation. In that case there is no invitation ID to record.
+		if inv != nil {
+			inUser.invitationID = inv.ID
+			ghInvites = append(ghInvites, inUser)
+		}
 	}
 
 	for _, l := range remove {