Here's my behavior:

  strstr(NULL, /* valid string */)

I can't find the needle in the haystack (actually, I can't find anything in the haystack. I can't find the haystack.) Thus I return NULL.

  strstr(/* valid string */, NULL)

I can't find the needle in the haystack (actually, I wouldn't be able to find it: I don't know what I'm looking for.) Return NULL.