Leave current value if var is empty in a UPDATE query (sql)

I want not update a record when the variable is empty when executing a UPDATE query in SQL. However, when the variable is filled, the record should be updated.

So in example, the myAge field in the database have currently a value of 20 (type int).
After executing the following query, the record should still be 20.

$age = '';

db_con->query("UPDATE info SET myAge = ".$age." WHERE account_id = 1");

Ps: I know I could check if the variable is empty with PHP, but I was wondering If this could be archieved within SQL?