[thermostat] Fix on_boot_restore_from DEFAULT_PRESET validation bypass (#15383)

esphome/components/thermostat/climate.py

@@ -503,7 +503,7 @@ def validate_thermostat(config):
     # If restoring default preset on boot is true then ensure we have a default preset
     if (
         CONF_ON_BOOT_RESTORE_FROM in config
-        and config[CONF_ON_BOOT_RESTORE_FROM] is OnBootRestoreFrom.DEFAULT_PRESET
+        and config[CONF_ON_BOOT_RESTORE_FROM] == "DEFAULT_PRESET"
         and CONF_DEFAULT_PRESET not in config
     ):
         raise cv.Invalid(