SELECT 
  c.parent_id, 
  sm.search_phrases, 
  c.category_id 
FROM 
  cscart_categories AS c 
  LEFT JOIN cscart_ab__search_motivation AS sm ON sm.category_id = c.category_id 
  AND sm.lang_code = 'ru' 
WHERE 
  c.id_path LIKE "%303%" 
  AND (
    sm.search_phrases != "NULL" 
    OR c.level = (
      SELECT 
        level 
      FROM 
        cscart_categories 
      WHERE 
        category_id = 303
    )
  ) 
  AND c.company_id = 0 
  AND c.status = "A"

Query time 0.00108

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "c",
      "access_type": "ALL",
      "rows": 654,
      "filtered": 100,
      "attached_condition": "c.company_id = 0 and c.id_path like '%303%' and c.`status` = 'A'"
    },
    "table": {
      "table_name": "sm",
      "access_type": "ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["category_id"],
      "ref": ["admin_test.c.category_id"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "trigcond(sm.search_phrases <> 'NULL' or c.`level` = (subquery#2)) and trigcond(sm.lang_code = 'ru')"
    },
    "subqueries": [
      {
        "query_block": {
          "select_id": 2,
          "table": {
            "table_name": "cscart_categories",
            "access_type": "const",
            "possible_keys": ["PRIMARY", "p_category_id"],
            "key": "PRIMARY",
            "key_length": "3",
            "used_key_parts": ["category_id"],
            "ref": ["const"],
            "rows": 1,
            "filtered": 100
          }
        }
      }
    ]
  }
}