tree-wide: drop NULL sentinel from strjoin

This makes strjoin and strjoina more similar and avoids the useless final
argument.

spatch -I . -I ./src -I ./src/basic -I ./src/basic -I ./src/shared -I ./src/shared -I ./src/network -I ./src/locale -I ./src/login -I ./src/journal -I ./src/journal -I ./src/timedate -I ./src/timesync -I ./src/nspawn -I ./src/resolve -I ./src/resolve -I ./src/systemd -I ./src/core -I ./src/core -I ./src/libudev -I ./src/udev -I ./src/udev/net -I ./src/udev -I ./src/libsystemd/sd-bus -I ./src/libsystemd/sd-event -I ./src/libsystemd/sd-login -I ./src/libsystemd/sd-netlink -I ./src/libsystemd/sd-network -I ./src/libsystemd/sd-hwdb -I ./src/libsystemd/sd-device -I ./src/libsystemd/sd-id128 -I ./src/libsystemd-network --sp-file coccinelle/strjoin.cocci --in-place $(git ls-files src/*.c)

git grep -e '\bstrjoin\b.*NULL' -l|xargs sed -i -r 's/strjoin\((.*), NULL\)/strjoin(\1)/'

This might have missed a few cases (spatch has a really hard time dealing
with _cleanup_ macros), but that's no big issue, they can always be fixed
later.

src/debug-generator/debug-generator.c

@@ -113,7 +113,7 @@ static int generate_mask_symlinks(void) {
         STRV_FOREACH(u, arg_mask) {
                 _cleanup_free_ char *p = NULL;
 
-                p = strjoin(arg_dest, "/", *u, NULL);
+                p = strjoin(arg_dest, "/", *u);
                 if (!p)
                         return log_oom();
 
@@ -136,7 +136,7 @@ static int generate_wants_symlinks(void) {
         STRV_FOREACH(u, arg_wants) {
                 _cleanup_free_ char *p = NULL, *f = NULL;
 
-                p = strjoin(arg_dest, "/", arg_default_unit, ".wants/", *u, NULL);
+                p = strjoin(arg_dest, "/", arg_default_unit, ".wants/", *u);
                 if (!p)
                         return log_oom();