if(req.body.emName){
// This code block will run when req.body.emName contains value and is not undefined or null
let sql = `UPDATE details SET name = CASE WHEN '${req.body.emName}' THEN ${req.body.emName} END where id=${req.body.emId}`;
}else{
// req.body.emName is either empty/undefined or null
// ...
}