The format_date function tries to fill the (non-POSIX) tm_zone field

of struct tm, assuming the corresponding argument is a string.  By the
scsh documentation, #f is allowed, too.  So just use NULL in that case
instead of raising an argument-type exception, as was the case
previously.

scsh/time1.c

@@ -333,7 +333,8 @@ s48_value format_date(s48_value sch_fmt, s48_value sch_sec, s48_value sch_min,
     d.tm_isdst = (S48_EQ_P (sch_summer, S48_FALSE)) ? 0 : 1;
 
 #ifdef HAVE_STRUCT_TM_TM_ZONE
-    d.tm_zone = s48_extract_string(sch_tz); /* FreeBSD's strftime reads this */
+    /* FreeBSD's strftime reads this */
+    d.tm_zone = (S48_EQ_P(sch_tz, S48_FALSE)) ? NULL : s48_extract_string(sch_tz);
 #endif
 
     /* Copy fmt -> fmt2, converting ~ escape codes to % escape codes.