Fix plot.PrepGR when all Qobs are missing

The plot.PrepGR() function does not run if Qobs = NA.

data(L0123001, package = "airGR")
prep <- PrepGR(DatesR = BasinObs$DatesR, Precip = BasinObs$P,
               PotEvap = BasinObs$E, Qobs = NA, HydroModel = "GR4J")

plot(prep)

Error in plot.window(...) : valeurs finies requises pour 'ylim'
De plus : Warning messages:
1: In min(x) : aucun argument trouvé pour min ; Inf est renvoyé
2: In max(x) : aucun argument pour max ; -Inf est renvoyé

It could be better it to display the following text in the second plot: "No observed discharges".