Can $E_{cell}^o$ or $\Delta_r G^o$ for a cell reaction ever be equal to zero?

What will be the $EMF$ of the cell formed by the following half-cells in $V$?
$Mg^{2+} + 2e^- \to Mg_{(s)}; E = -2.37 \ V$
$Cu^{2+} + 2e^- \to Cu_{(s)}; E = +0.33 \ V$

The electrode potential for $M^{2+}_{(aq)} + e^- \rightarrow M^{+}_{(aq)}$ and $M^{+}_{(aq)} + e^- \rightarrow M_{(s)}$ are $+0.15 \text{ V}$ and $+0.50 \text{ V}$ respectively. The value of $E^\circ_{M^{2+}/M}$ will be: (in $\text{ V}$)

The standard hydrogen electrode has a zero electrode potential because .....

For the cell reaction $Mg_{(s)} + Zn^{2+}_{(aq)} (1M) \rightarrow Zn_{(s)} + Mg^{2+}_{(aq)} (1M)$,the $emf$ is $1.60 \ V$. The $E$ of the cell is ........ $V$.

What is the standard reduction potential $(E^o)$ for $Fe^{3+} \to Fe$ ? ............... $V$
Given that :
$Fe^{2+} + 2e^- \to Fe;$ $E^o_{Fe^{2+}/Fe} = -0.47 \ V$
$Fe^{3+} + e^- \to Fe^{2+};$ $E^o_{Fe^{3+}/Fe^{2+}} = +0.77 \ V$