Add nullptr check in Equals method to handle null impl_ pointers

Author: mroz45

cpp/src/arrow/compute/expression.cc

@@ -220,6 +220,7 @@ void PrintTo(const Expression& expr, std::ostream* os) {
 
 bool Expression::Equals(const Expression& other) const {
   if (Identical(*this, other)) return true;
+  if (impl_ == nullptr || other.impl_ == nullptr) return false;
 
   if (impl_->index() != other.impl_->index()) {
     return false;

cpp/src/arrow/compute/expression_test.cc

@@ -1916,6 +1916,12 @@ TEST(Expression, SerializationRoundTrips) {
                          equal(field_ref("beta"), literal(3.25f))}));
 }
 
+TEST(Expression, ExpressionEquals) {
+  Expression expr1;
+  Expression expr2(literal(1));
+
+  EXPECT_FALSE(expr1.Equals(expr2));
+}
 TEST(Projection, AugmentWithNull) {
   // NB: input contains *no columns* except i32
   auto input = ArrayFromJSON(struct_({kBoringSchema->GetFieldByName("i32")}),