cocci: strbuf.buf is never NULL

We recently noticed one old code from 19 years ago protecting
against an ancient strbuf convention that the .buf member can be
NULL for an empty strbuf.  As that is no longer the case in the
modern codebase, let's catch such a construct.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: gitster

contrib/coccinelle/strbuf.cocci

@@ -60,3 +60,10 @@ expression E1, E2;
 @@
 - strbuf_addstr(E1, real_path(E2));
 + strbuf_add_real_path(E1, E2);
+
+// In modern codebase, .buf member of an empty strbuf is not NULL.
+@@
+struct strbuf SB;
+@@
+- SB.buf ? SB.buf : ""
++ SB.buf