Don't try and encrypt password if none passed (#459)

## Status

- Ready for review

## What's changed?

- Only try and decrypt the password if a password is passed, since
resetting and updating student details use the same endpoint

## Steps to perform after deploying to production

N/A

Author: danhalson

lib/concepts/school_student/update.rb

@@ -17,9 +17,9 @@ def call(school:, student_id:, school_student_params:, token:)
 
       def update_student(school, student_id, school_student_params, token)
         username = school_student_params.fetch(:username, nil)
-        encrypted_password = school_student_params.fetch(:password, nil)
-        password = DecryptionHelpers.decrypt_password(encrypted_password)
         name = school_student_params.fetch(:name, nil)
+        password = school_student_params.fetch(:password, nil)
+        password = DecryptionHelpers.decrypt_password(password) if password.present?
 
         validate(username:, password:, name:)