Extend std::optional null regression coverage (assignment, get_to)

Adds two cases to the existing null-handling regression test for
std::optional<T>: assignment (opt = json_null) throws for the same
reason as direct construction, and get_to() correctly yields
std::nullopt. Prompted by a review comment on #5246 asking for a
fuller regression matrix; construction and get<optional<T>>() were
already covered by #5247.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-conversions.cpp

@@ -1782,6 +1782,17 @@ TEST_CASE("std::optional")
                              "[json.exception.type_error.302] type must be string, but is null", json::type_error&);
         CHECK_THROWS_WITH_AS(std::optional<int>(j_null),
                              "[json.exception.type_error.302] type must be number, but is null", json::type_error&);
+
+        // Assignment goes through the same overload resolution as direct
+        // construction, so it throws for the same reason.
+        std::optional<std::string> opt_assign;
+        CHECK_THROWS_WITH_AS(opt_assign = j_null,
+                             "[json.exception.type_error.302] type must be string, but is null", json::type_error&);
+
+        // get_to() is the correct way to obtain std::nullopt from a JSON null.
+        std::optional<std::string> opt_get_to = "placeholder";
+        j_null.get_to(opt_get_to);
+        CHECK(opt_get_to == std::nullopt);
     }
 
     SECTION("string")