Fix compilation error for models with events and xdot=0 (#2111)

Fixes #2110

Author: dweindl

python/sdist/amici/de_export.py

@@ -1999,9 +1999,10 @@ def _compute_equation(self, name: str) -> None:
 
                 # need to check if equations are zero since we are using
                 # symbols
-                if not smart_is_zero_matrix(self.eq("stau")[ie]):
+                if not smart_is_zero_matrix(self.eq("stau")[ie]) \
+                        and not smart_is_zero_matrix(self.eq("xdot")):
                     tmp_eq += smart_multiply(
-                        (self.sym("xdot_old") - self.sym("xdot")),
+                        self.sym("xdot_old") - self.sym("xdot"),
                         self.sym("stau").T,
                     )