No, because even NULL in C could mean NULL pointer, which is closer to undef than to an empty string.

Quite true, in which case the appropriate phrase would be 'NULL string pointer', or maybe 'NULL pointer to a string', but that's not how the question is stated. Now I understand the need to be sensitive to context when interpreting a question, however when I first read your post I thought you were giving a general answer suitable to a well-phrased query. If this whole debate turns on your interlocuter not knowing precise terminology, then there's really no point to this.