fix!: omit name, avatar_url and last_seen_at from responses when empty (#18005)

User name, avatar URL, and last seen at, are not required fields so they
can be empty. Instead of returning the 0 values from Go, we want to make
it more agnostic, and omit them when they are empty. This make the docs
and usage way clearer for consumers.

cli/testdata/coder_users_list_--output_json.golden

@@ -2,7 +2,6 @@
   {
     "id": "==========[first user ID]===========",
     "username": "testuser",
-    "avatar_url": "",
     "name": "Test User",
     "email": "testuser@coder.com",
     "created_at": "====[timestamp]=====",
@@ -23,8 +22,6 @@
   {
     "id": "==========[second user ID]==========",
     "username": "testuser2",
-    "avatar_url": "",
-    "name": "",
     "email": "testuser2@coder.com",
     "created_at": "====[timestamp]=====",
     "updated_at": "====[timestamp]=====",