[FIX] website: show a label for empty selection values

Before this commit, if the user tried to setup a required selection
field whose first value is empty, it was possible but the empty value
appeared as a 0px-height div in the DOM preview. That empty value also
shows up when trying to use the conditionally-hidden fields feature: if
another field's visibility depends on such selection field, the editor
UI was visually broken as the first choice was an empty option element.

Now, we show "<no value>" in both cases.

Note: this is not done in earlier versions than 15.0 although the
problem exists, but it was decided that the real problem was about the
visibility select being visually broken and that feature only appeared
in 15.0.

Related to opw-3003952

Part-of: odoo/odoo#105870