[IMP] Server env default fields string

Not all fields have their string in the args, but '.string' returns the computed (based on field name) string if not set in the args.

server_environment/models/server_env_mixin.py

@@ -401,7 +401,7 @@ class ServerEnvMixin(models.AbstractModel):
             base_field_cls = base_field.__class__
             field_args = base_field.args.copy() if base_field.args else {}
             field_args.pop("_sequence", None)
-            fieldlabel = "{} {}".format(field_args.get("string", ""), "Env Default")
+            fieldlabel = "{} {}".format(base_field.string or "", "Env Default")
             field_args.update(
                 {
                     "sparse": "server_env_defaults",