Hide bottom space for error message when validation is None

[falkoschindler]

PR #3446 added a bottom space for the error message of validation messages. This turned out to be unnecessary when no validation is used. This PR changes the behavior to only reserve the bottom space if the validation property is not None.

Code for experimenting with the new implementation:

input_ = ui.input(value='Hello, world!').classes('border')

def add_validation():
    input_.validation = {
        'Must start with "Hello"': lambda x: x.startswith('Hello'),
        'Must end with "world!"': lambda x: x.endswith('world!')
    }

def remove_validation():
    input_.validation = None

ui.button('Add validation', on_click=add_validation)
ui.button('Remove validation', on_click=remove_validation)