Guard the null value of a value-initialized iterator; drop the unneeded found flag

With the root's byte range, diagnostics() reaches get_byte_positions() also
with JSON_DIAGNOSTICS when there is no value, as for a value-initialized
iterator (invalid_iterator.212). get_byte_positions() now returns no range
for a null value, the same change as #6003, so either can be merged first.

In the view, a value that value_path() does not find leaves no tokens, so the
found flag added nothing to the tokens.empty() test.

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

single_include/nlohmann/json_view.hpp

@@ -5097,8 +5097,9 @@ std::string diagnostics(const error_site site)
     std::string context;
 #if JSON_DIAGNOSTICS
     std::vector<std::string> tokens;
-    const bool found = edited ? value_path<true>(d, site.value(), tokens) : value_path<false>(d, site.value(), tokens);
-    if (found && !tokens.empty())
+    // a value that is not found leaves no tokens, like the root
+    static_cast<void>(edited ? value_path<true>(d, site.value(), tokens) : value_path<false>(d, site.value(), tokens));
+    if (!tokens.empty())
     {
         // the root value has no path, but still has a byte range
         std::string path;