json_view: return the empty context so clang can elide its copy

clang 22 with -Weverything (ci_test_standards_clang) reports -Wnrvo for
'return context;' in diagnostics(), because the early 'return "";' after
context is declared rules out the named return value optimization. context is
still empty there, so return it instead.

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

single_include/nlohmann/json_view.hpp

@@ -4765,7 +4765,8 @@ std::string diagnostics(const error_site site)
     const bool found = edited ? value_path<true>(d, site.value(), tokens) : value_path<false>(d, site.value(), tokens);
     if (!found || tokens.empty())
     {
-        return "";
+        // context is still empty; returning it keeps the copy elided (-Wnrvo)
+        return context;
     }
     std::string path;
     for (const std::string& token : tokens)