Fix EQ constraint in case no hydro exist

scripts/solve_network.py

@@ -216,6 +216,7 @@ def add_EQ_constraints(n, o, scaling=1e-1):
         .T.groupby(ggrouper, axis=1)
         .apply(join_exprs)
     )
+    if not n.storage_units_t.inflow.empty:
         lhs_spill = (
             linexpr(
                 (
@@ -228,6 +229,8 @@ def add_EQ_constraints(n, o, scaling=1e-1):
         )
         lhs_spill = lhs_spill.reindex(lhs_gen.index).fillna("")
         lhs = lhs_gen + lhs_spill
+    else:
+        lhs = lhs_gen
     define_constraints(n, lhs, ">=", rhs, "equity", "min")