A standard select/dropdown form option allows selecting any options, which is usually fine unless you have a choice option such as 'Make your selection', 'Pick One' etc. These options typically come as the first select options and also don't have a 'value'. If the user does not make a selection, the value will be blank even if the option is required. The way to resolve this issue is to use the 'disabled' HTML syntax for the form option.
In this example, we also ensure that the first choice 'value' is blank/null and that it is also 'selected' so that it shows the select instruction first.