Guard optional assignment test behind JSON_USE_IMPLICIT_CONVERSIONS

opt_assign = j_null relies on basic_json's implicit conversion
operator to satisfy std::optional's assignment operator template
(which requires is_assignable, not just is_constructible). With
implicit conversions disabled the operator is explicit, so the
assignment doesn't compile at all rather than throwing at runtime.
Fixes the ci_test_noimplicitconversions failure on #5269.

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

tests/src/unit-conversions.cpp

@@ -1784,10 +1784,14 @@ TEST_CASE("std::optional")
                              "[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.
+        // construction, so it throws for the same reason. This relies on
+        // basic_json's implicit conversion operator, so it only applies
+        // when JSON_USE_IMPLICIT_CONVERSIONS is enabled (the default).
+#if JSON_USE_IMPLICIT_CONVERSIONS
         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&);
+#endif
 
         // get_to() is the correct way to obtain std::nullopt from a JSON null.
         std::optional<std::string> opt_get_to = "placeholder";