Fix three assertion violations in nseq string solver (#9106)

- seq_model.cpp: skip trivial memberships in collect_var_regex_constraints;
  SAT leaf nodes can have "" in nullable_regex (trivial) in addition to
  primitive (single-variable) memberships after Brzozowski derivative
  consumption reduces a concrete string membership to empty.

- seq_nielsen.cpp: fix SASSERT(!var) typo in var_ub(); should be SASSERT(var)
  matching the pattern in var_lb().

- seq_regex.cpp: replace VERIFY(re_expr) with null guard in
  minterm_to_char_set(); nullptr means no regex constraint and should
  return the full alphabet, as the test test_minterm_nullptr_is_full expects.


Agent-Logs-Url: https://github.com/Z3Prover/z3/sessions/31db5346-9b60-4a20-a101-beca9fc9e4f8

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>

src/smt/seq/seq_nielsen.cpp

@@ -310,7 +310,7 @@ namespace seq {
     }
 
     unsigned nielsen_node::var_ub(euf::snode* var) const {
-        SASSERT(!var);
+        SASSERT(var);
         unsigned v = UINT_MAX;
         m_var_ub.find(var->id(), v);
         return v;