Use the github username as the name, if no name is set. (#9388)

The UI renders that value
[here](https://github.com/buildbuddy-io/buildbuddy/blob/master/enterprise/app/org/org_members.tsx#L253)
Fixes
[this](https://buildbuddy-corp.slack.com/archives/C0495TM9UUE/p1747423662459899)

---------

Co-authored-by: Brandon Duffany <brandon@buildbuddy.io>

Author: maggie-lou

server/tables/tables.go

@@ -312,12 +312,17 @@ func (u *User) TableName() string {
 }
 
 func (u *User) ToProto() *uspb.DisplayUser {
+	name := strings.TrimSpace(u.FirstName + " " + u.LastName)
+	// Use the github username as the name, if no name is set.
+	if name == "" && strings.HasPrefix(u.SubID, "https://github.com/") {
+		name = strings.TrimPrefix(u.SubID, "https://github.com/")
+	}
 	return &uspb.DisplayUser{
 		UserId: &uspb.UserId{
 			Id: u.UserID,
 		},
 		Name: &uspb.Name{
-			Full:  strings.TrimSpace(u.FirstName + " " + u.LastName),
+			Full:  name,
 			First: u.FirstName,
 			Last:  u.LastName,
 		},