[cpp.concat] p3 says

If the result is not a valid preprocessing token, the behavior is undefined.

#define concat(X,Y) X##Y
concat(,)   // #1

The result of concat(,) is a single placemarker preprocessing token(result from the concatenation of two placemarker preprocessing tokens).

[lex.pptoken] specifies all normative preprocessing tokens, which does not mention placemarker preprocessing token. The note also says that:

Placemarker preprocessing tokens do not appear in the syntax because they are temporary entities that exist only within translation phase 4.

It is unclear whether the "valid preprocessing token" refers to the preprocessing tokens defined in [lex.pptoken] or refers to the tokens in [lex.pptoken] and the placemarker preprocessing token. Presumably, it refers to the latter. Furhtermore, the text "preprocessing token" in [cpp] intends to include the syntax preprocessing tokens and the placemarker preprocessing token.

[cpp.concat] p3 may be clearer if it is changed to

If the result is not a valid preprocessing token(include placemarker preprocessing token), the behavior is undefined.