builtin/config.c: Fix a sparse warning

builtin/config.c: Fix a sparse warning

Sparse issues an "Using plain integer as NULL pointer" warning while
checking a 'struct strbuf_list' initializer expression. The initial
field of the struct has pointer type, but the initializer expression
is given as '{0}'. In order to suppress the warning, we simply replace
the initializer with '{NULL}'.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Jeff King <peff@peff.net>