Address the clang-tidy findings of the comparisons

Separate the comparison of discarded values from the other types, so
that the conditional chain has no repeated branch bodies, and mark
the deliberate comparisons of views with empty containers in the
tests.

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

tests/src/unit-json_view.cpp

@@ -1252,9 +1252,9 @@ TEST_CASE("json_view comparison")
 
         // discarded values compare as basic_json's do
         const json discarded(json::value_t::discarded);
-        CHECK((json_view() == json_view()) == (discarded == discarded));
+        CHECK((json_view() == json_view()) == (discarded == discarded)); // NOLINT(readability-container-size-empty): operator== is tested
         CHECK((json_view() == discarded) == (discarded == discarded));
-        CHECK(!(json_view() == json_document::parse("null").root()));
+        CHECK(!(json_view() == json_document::parse("null").root())); // NOLINT(readability-container-size-empty)
         CHECK(!(json_document::parse("null").root() == discarded));
     }