Keep the byte range in exceptions about the root value (#5987)

With JSON_DIAGNOSTICS and JSON_DIAGNOSTIC_POSITIONS both set, exception::diagnostics()
and the json_view diagnostics() returned early for a value without a path (the
root), dropping "(bytes a-b)". Build the "(path) " prefix only when there are
tokens and always append the byte range.

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

docs/mkdocs/docs/api/macros/json_diagnostic_positions.md

@@ -35,6 +35,9 @@ Note that enabling this macro increases the size of every JSON value by two `std
 overhead to parsing, copying JSON value objects, and the generation of error messages for exceptions. It also causes
 these values to be reported in those error messages.
 
+If [`JSON_DIAGNOSTICS`](json_diagnostics.md) is also set, the message contains the path and the byte range, such as
+`(/1) (bytes 4-8)`. An exception about the root value has no path and only the range, such as `(bytes 0-4)`.
+
 ## Default definition
 
 The default value is `0` (position diagnostics are switched off).