t7518: fix flaky grep invocation

t7518: fix flaky grep invocation

t7518.1 added in commit 862e80a413 ("ident: handle NULL email when
complaining of empty name", 2017-02-23), was trying to make sure that
the test with an empty ident did not segfault and did not result in
glibc quiety translating a NULL pointer into a name of "(null)".  It did
the latter by ensuring that a grep for "null" didn't appear in the
output, but on one automatic CI run I observed the following output:

fatal: empty ident name (for <runner@fv-az128-670.gcliasfzo2nullsdbrimjtbyhg.cx.internal.cloudapp.net>) not allowed

Note that 'null' appears as a substring of the domain name, found
within 'gcliasfzo2nullsdbrimjtbyhg'.  Tighten the test by searching for
"(null)" rather than "null".

Signed-off-by: Elijah Newren <newren@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>