Address the clang-tidy findings of element access and iteration

Marks the default initializer of the item's index string (needed by GCC's
-Weffc++) and, in the test, an escaped literal and a comparison of find()
with end(), which is what the test is about.

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

include/nlohmann/detail/view/iterator.hpp

@@ -183,7 +183,7 @@ class view_item
 
   private:
     iterator m_it;
-    std::string m_index{};
+    std::string m_index{}; // NOLINT(readability-redundant-member-init)
 };
 
 /// the range returned by basic_json_view::items()