Fix strftime/1 and strflocaltime/1 to allow an empty result (#3670)

These functions failed with "unknown system failure" whenever strftime()
returned 0 for a non-empty format, but 0 also means the format expands to
an empty string, as %p does in a locale without AM/PM.

Prepend a character to the format and trim it from the result, so that 0
can only mean the result is too long. Now 0 | strflocaltime("%p") gives
"" in fr_FR.UTF-8, and a result that is still too long fails with
"result too long" error instead. The size limit is unchanged.

tests/jq.test

@@ -1945,6 +1945,22 @@ try ["OK", strflocaltime({})] catch ["KO", .]
 0
 ["KO","strflocaltime/1 requires a string format"]
 
+[strftime("", " %Y ")]
+0
+["", " 1970 "]
+
+try strftime("%c" * 10) catch .
+0
+"strftime/1: result too long"
+
+[strflocaltime("", " %Y ")]
+[2015,2,5,23,51,47,4,63]
+["", " 2015 "]
+
+try strflocaltime("%c" * 10) catch .
+[2015,2,5,23,51,47,4,63]
+"strflocaltime/1: result too long"
+
 [strptime("%Y-%m-%dT%H:%M:%SZ")|(.,mktime)]
 "2015-03-05T23:51:47Z"
 [[2015,2,5,23,51,47,4,63],1425599507]