LBU (Le Bon Usage) treats this question partially au 373 c and speaks of "redondance expressive".
Un terme est mis en évidence au début ou à la fin de la phrase,
et un pronom personnel ou démonstratif occupe la place normale de ce terme :
• Sujets (§ 237, b) : Hugo, toujours gigantesque, s'il vient à succéder à
Lemercier dans l'Académie, IL a l'air de succéder à Napoléon, tant il en
parle tout d'abord (S.-BEUVE, Mes poisons, p. 48). — Par bonheur, une
tête de vipère, C'est triangulaire [...]. Par bonheur, une peau de vipère,
c'est rugueux (H. BAZIN, Vipère au poing, I). [C'est le pronom placé
devant le verbe qui détermine l'accord.] — IL avait beaucoup changé,
Camus (SARTRE, Situation X, p. 196). [Texte d'une interview.] —
ÇA ne va pas, la tête ? formule très familière équivalant à Vous
déraisonnez. — Moi, je vous baptise avec l'eau ; mais IL vient, celui qui
est plus puissant que moi (Bible, trad. CRAMPON, Luc, III, 16). [ Q
• Attributs (§ 247, d) : Libre, je ne LE suis à peu près jamais (PROUST)
(user LPH's bold type)
The information provided by "Le Bon Usage" leaves out a lot and that results from my point of view, in misrepresentation. Here is what I can deduce. It is a good idea for those really curious about this question of "redondance expressive/dislocation" to take note of the following question, as it is closely related: Sujet+pronom à la place du sujet seul.
In the printed material you can't find "C'est nul les…" but you do find "C'est beau les …" . The first form is not to be construed as being the same as the second. The first does not mean "Les examens sont nuls.", which means nothing. Instead it means "Le système des examens est un système qui ne vaut rien.". This is modern French parlance of very recent origin and the redundancy or dislocation as others call it, is not expressive; it is so little expressive that nobody is able to explain what is this expressivity. The second form is of the type claimed ; it does mean "Les hommes sont beaux.". The question is why is there not the same nuance in the plain form. The answer, I believe, is that a plain assertion does not correspond to reality : yes, there are men who are beautiful but there are also men who are not so beautiful; that is common knowledge and so the assertion hits one in the face as a flat statement that would have us redefine reality, an assertion that presents a new axiom. Instead, that is not what the author has in mind : he is concerned with the transcendence of that possible beauty in a certain number of the elements of the species to make it rather a characteristic of the species as a whole, to say that beauty is inherent to that species : "On the whole, a man is a beautiful creature.", "There is beauty in the human species.". This English translation shows a certain symmetry: the expected subject is rejected at the end, an adverb (there) is used to introduce the central concept (beauty). That is what the expressiveness consists in and some will probably be able to add complements to that and/or make adjustments.
It must be reckoned that not all forms on the morphological model of "expressive redondance" really belong to the category; it is only justified through special contexts that good writers are more apt to discern. There are nowadays people aware of the fact that this redundancy can be nothing but childish speech (effet de style ou faute de français).
The form "Les examens c'est nul." shows that there is nothing in it but approximate recoding of the normal meaning of words, that being so out of an unability to form coherent thoughts, whatever the reason. We all know that such a way of expressing the idea wouldn't do in a serious discussion. The "expressive redundancy" is inexistent; the utterance is merely a case of childish speech as characterised by the philosophers and journalists mentioned in the article shown above, an inchoate attempt at normal expression.