string repeat of an empty string is an error

Fixes fish-shell#3898

Author: krader1961

src/builtin_string.cpp

@@ -1046,7 +1046,7 @@ static int string_repeat(parser_t &parser, io_streams_t &streams, int argc, wcha
     wcstring storage;
     bool is_empty = true;
 
-    if ((to_repeat = string_get_arg(&i, argv, &storage, streams)) != NULL) {
+    if ((to_repeat = string_get_arg(&i, argv, &storage, streams)) != NULL && *to_repeat) {
         const wcstring word(to_repeat);
         const bool rep_until = (0 < max && word.length()*count > max) || !count;
         const wcstring repeated = rep_until ? wcsrepeat_until(word, max) : wcsrepeat(word, count);

tests/string.in

@@ -132,4 +132,12 @@ echo 'stdin' | string repeat -n1 'and arg'; or echo "exit 2"
 
 string repeat -n; or echo "exit 2"
 
-string repeat -l fakearg 2>&1 | head -n1 1>&2
+string repeat -l fakearg 2>&1 | head -n1 1>&2
+
+string repeat ''
+and echo string repeat empty string did not fail
+
+string repeat -n3 ''
+and echo string repeat empty string did not fail
+
+exit 0