It's true that, with phpMyAdmin, there would be no way to tell the difference in the display of a NULL field and the display of a (say) varchar(8) field that contains the text string 'NULL', but I'm sure that the creator of phpMyAdmin figured (rightly so) that there's pretty darned close to a zero chance that anybody would actually store 'NULL' in a field's value. (Though they maybe reckoned without considering amateurs who don't know the difference.)
Personally, when I display NULL in a table dump, I do so using a different text color than I use for normal text, which then makes it clear.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.