It sounds like your TRUIE variable uniquely identifies each experimental unit. If that is correct and you just want an overall mean estimate (which assumes all units come from the same population), then the intercept in an intercept-only model estimates it. Of course, in that case you don't really need to fit a model and can just use PROC MEANS.
proc mixed data=fetus_truie;
model nbr_pig= / s;
run;
proc means data=fetus_truie mean stderr;
var nbr_pig;
run;
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