[frontend] use input data without `None` fields
    
    to transform only the specified values to the WTForms, so they
    can correctly use the default values for the rest (e.g. when
    `spec_template` is not set).
    
    Also, there is some issue when passing an existing package object
    (or its data) to the form in order to use it as defaults, so I am
    workarounding it.
    
    It turns out that the whole JSON --> WTForms adapter code wasn't
    written very well and is more patchy and harder to use,
    than it needs to be. We should address that.