cryptsetup: avoid calling strv_find() on a NULL pointer

When the header= option comes before any other type= defining one, we
trip over an assertion:

Jun 04 15:45:33 H testsuite-24.sh[752]: + systemctl start systemd-cryptsetup@detached.service
Jun 04 15:45:33 H systemd[1]: Starting systemd-cryptsetup@detached.service...
Jun 04 15:45:33 H systemd-cryptsetup[4641]: Assertion 'name' failed at src/basic/strv.c:21, function strv_find(). Aborting.
...
Jun 04 15:45:33 H systemd-coredump[4643]: Process 4641 (systemd-cryptse) of user 0 dumped core.
...
                                          Stack trace of thread 4641:
                                          #0  0x00007ff9256afe5c __pthread_kill_implementation (libc.so.6 + 0x8ce5c)
                                          #1  0x00007ff92565fa76 raise (libc.so.6 + 0x3ca76)
                                          #2  0x00007ff9256497fc abort (libc.so.6 + 0x267fc)
                                          #3  0x00007ff926076047 log_assert_failed (libsystemd-shared-253.so + 0x276047)
                                          #4  0x00007ff9260ab317 strv_find (libsystemd-shared-253.so + 0x2ab317)
                                          #5  0x0000000000405927 parse_one_option (systemd-cryptsetup + 0x5927)
                                          #6  0x0000000000407793 parse_options (systemd-cryptsetup + 0x7793)
                                          #7  0x000000000040fa0c run (systemd-cryptsetup + 0xfa0c)
                                          #8  0x000000000041137f main (systemd-cryptsetup + 0x1137f)
                                          #9  0x00007ff92564a510 __libc_start_call_main (libc.so.6 + 0x27510)
                                          #10 0x00007ff92564a5c9 __libc_start_main@@GLIBC_2.34 (libc.so.6 + 0x275c9)
                                          #11 0x0000000000403915 _start (systemd-cryptsetup + 0x3915)
                                          ELF object binary architecture: AMD x86-64

src/cryptsetup/cryptsetup.c

@@ -234,7 +234,7 @@ static int parse_one_option(const char *option) {
                         return log_oom();
 
         } else if ((val = startswith(option, "header="))) {
-                if (!STR_IN_SET(arg_type, ANY_LUKS, CRYPT_LUKS1, CRYPT_LUKS2, CRYPT_TCRYPT))
+                if (!arg_type || !STR_IN_SET(arg_type, ANY_LUKS, CRYPT_LUKS1, CRYPT_LUKS2, CRYPT_TCRYPT))
                         arg_type = ANY_LUKS;
 
                 if (!path_is_absolute(val))