scripts/t: Use is() instead of ok() to test for undef

scripts/t/Dpkg_Deps.t

@@ -197,7 +197,7 @@ my $dep_bad_arch = deps_parse('package [alpha] [amd64]');
 is($dep_bad_arch, undef, 'Bogus repeated arch restriction');
 
 my $dep_bad_multiline = deps_parse("a, foo\nbar, c");
-ok(!defined($dep_bad_multiline), 'invalid dependency split over multiple line');
+is($dep_bad_multiline, undef, 'invalid dependency split over multiple line');
 
 delete $SIG{__WARN__};
 

scripts/t/Dpkg_Path.t

@@ -55,6 +55,6 @@ is(relative_to_pkg_root("$tmpdir/a/b/c"), 'b/c', 'relative_to_pkg_root');
 
 chdir($tmpdir);
 
-ok(!defined(get_pkg_root_dir('debian/a/b/c')), 'get_pkg_root_dir undef');
-ok(!defined(relative_to_pkg_root('debian/a/b/c')), 'relative_to_pkg_root');
+is(get_pkg_root_dir('debian/a/b/c'), undef, 'get_pkg_root_dir undef');
+is(relative_to_pkg_root('debian/a/b/c'), undef, 'relative_to_pkg_root undef');
 is(guess_pkg_root_dir('debian/a/b/c'), 'debian/a', 'guess_pkg_root_dir fallback');