Report one-character non-numeric array indices like basic_json in the view

Develop's #5800 made basic_json report a JSON pointer array index such as
"/a" as parse_error.109 instead of out_of_range.404, as longer ones were;
the view's array_index() follows, with "-" and the empty token still
unresolved.

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

single_include/nlohmann/json_view.hpp

@@ -2551,7 +2551,9 @@ index_status array_index(const StringType& s, std::size_t& idx) noexcept
     {
         return index_status::leading_zero;
     }
-    if (s.size() > 1 && !(s[0] >= '1' && s[0] <= '9'))
+    // as basic_json: also a single character, but "-" and the empty token are
+    // left to the conversion below and reported as unresolved
+    if (!s.empty() && !(s.size() == 1 && s[0] == '-') && !(s[0] >= '0' && s[0] <= '9'))
     {
         return index_status::not_number;
     }