[LiveComponent] Handle loose comparison with empty placeholder

[FoxCie]

Fix for a bug that arises when using an empty placeholder in a required select field, that is wrongly treated as an absence of placeholder by LiveComponents.

Q	A
Bug fix?	yes
New feature?	no
Issues	Fix #2425
License	MIT

LiveComponents test placeholders of required selects to detect when to set a default value from the choices (i.e. if the select is required and does not have a placeholder, among other conditions) by just casting the placeholder to a boolean. This leads to empty placeholders (placeholders that are the empty string) to be treated as an absence of placeholders when they should not. This MR fixes this by replacing the condition on placeholders with a more appropriate one.
According to the docs https://symfony.com/doc/current/reference/forms/types/choice.html#placeholder and https://symfony.com/doc/current/reference/forms/types/choice.html#field-variables, the placeholder can be a boolean (false indicates that there should not be a placeholder), a string (the text to display for the empty value) or a TranslatableMessage (same as string, but goes through a translator before being displayed). The vars property is either the value of the field, or null if there is no placeholder specified. From this, I considered that there is no placeholder either if the placeholder holds false or null.

I added a simple test that checks the behaviour when there is a required select with an empty placeholder ; we expect the value of the field to be empty in such cases when we do not modify it.

#SymfonyHackday :)

[Kocal]

Nice catch @FoxCie, and thanks for the PR :)

[Kocal]

Thank you @FoxCie.

[FoxCie]

Thank you for the quick merge!