Fix heap buffer overflow when formatting an empty string

The `jv_string_empty` did not properly null-terminate the string data,
which could lead to a heap buffer overflow. The test case of
GHSA-p7rr-28xf-3m5w (`0[""*0]`) was fixed by the commit dc849e9bb74a,
but another case (`0[[]|implode]`) was still vulnerable. This commit
ensures string data is properly null-terminated, and fixes CVE-2025-48060.