Not sure if this doc might help a little for the one part.
As far as only updating the record if the user has actually input something it is that should be quite simple by by adding a condition to the right of the value.
Lets say you have your database update step and you want to only update the password if the user has entered something, then the column would be set to say password, the type will be text, the value would be your post vaiable from your form holding the password and the condition would be the same as the value, which means if user has input something then use the value.
Hope that makes sense. Here is a small sample, i use update conditions a lot.
