Validate fields

How validate field only we entered some value into input?

For example:
I have form with fields: username, password and email
I set 'value'=>'' in edit template for password input field
When I edit this fields, I want update filed password only when I
wrote some new password, but when I lost empty field password, this
field will be not updated. This works, but:
How validate field when only wrote into?

If field value is empty we not validate, but if field value is not
empty we get rules from $validate...

How do this?