UD-235: Added validation for update user password

@andrew00x @evoevodin 
<code>password.isEmpty()</code> is an excessive check.

WHY?
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If password is not <code>null</code> then client have already sent it so "Password required" is not correct message, but 
<i>"Password should contain at least 8 characters"</i> is! So you should not check password length twice.