Fix string.char() recording with no arguments.

(cherry picked from commit dfa692b)

`string.char()` call without arguments yields an empty string. JIT
recording machinery doesn’t handle this case. Each recording of a fast
function expects 1 result by default. Hence, when return from this call
is recorded, some garbage value is considered as a result to be yielded.
It is loaded from the corresponding slot as an IR with `IRT_NUM` type.
It leads to assertion failure in `rec_check_slots()`, when a next
bytecode is recorded, because type of TValue on the stack (`LJ_STR`)
isn't the same as IR (and TRef) type.

This patch handles the case without arguments by the loading of IR with
empty string reference as a result value.

Sergey Kaplun:
* added the description and the test for the problem

Resolves tarantool/tarantool#6371
Part of tarantool/tarantool#6548

Reviewed-by: Sergey Ostanevich <sergos@tarantool.org>
Reviewed-by: Igor Munkin <imun@tarantool.org>
Signed-off-by: Igor Munkin <imun@tarantool.org>

test/tarantool-tests/gh-6371-string-char-no-arg.test.lua

@@ -0,0 +1,29 @@
+local tap = require('tap')
+
+-- Test file to demonstrate assertion after `string.char()`
+-- recording.
+-- See also, https://github.com/tarantool/tarantool/issues/6371.
+
+local test = tap.test('gh-6371-string-char-no-arg')
+-- XXX: Number of loop iterations.
+-- * 1 -- instruction becomes hot.
+-- * 2 -- recording of the loop body.
+-- * 3 -- required for trace finalization, but this iteration
+--        runs the generated mcode and reproduces the issue.
+local NTEST = 3
+test:plan(NTEST)
+
+-- Storage for the results to avoid trace aborting by `test:ok()`.
+-- XXX: Use `table.new()` here to avoid side exit from trace by
+-- table resizing.
+local results = require('table.new')(3, 0)
+jit.opt.start('hotloop=1')
+for _ = 1, NTEST do
+  table.insert(results, string.char())
+end
+
+for i = 1, NTEST do
+  test:ok(results[i] == '', 'correct recording of string.char() without args')
+end
+
+os.exit(test:check() and 0 or 1)