[BUG] Batched multi-source JSON reader does not require each batch to contain at least one JSON line

[shrshi]

Describe the bug
This bug is related to #17058
There are two scenarios in which the partial table constructed from a batch i.e., byte range across sources can result in an empty table:

1. When the last batch is an incomplete row: The following test fails with the error Mismatch in JSON schema across batches in multi-source multi-batch JSON reader

TEST_F(JsonReaderTest, Debug)
{
  std::string data = R"(
  {"a": "b"}
  {"a": "b"}
  {"a": "b"}
  {"a": "b"}
  )";
  setenv("LIBCUDF_JSON_BATCH_SIZE", std::to_string(data.size() - 5).c_str(), 1);
  auto opts =
    cudf::io::json_reader_options::builder(cudf::io::source_info{data.data(), data.size()})
      .lines(true)
      .build();
  auto res = cudf::io::read_json(opts);
  unsetenv("LIBCUDF_JSON_BATCH_SIZE");
}

Since the penultimate batch reads the last row of the source, the byte range read in the last batch is empty and hence constructs an empty table.

2. When a batch contains a single JSON line of size > INT_MAX: In this case again, the partial table would be empty. However, the INT_MAX size constraint on the input buffer passed to the tokenizer precludes this error from appearing for now. A related corner case is documented in [BUG] Limit size of buffer read by batched multi-source JSON lines reader to be at most INT_MAX bytes #17058, where the batch contains multiple lines, the last of which is incomplete and results in a buffer of size greater than 2GB chars.

Expected behavior for the scenarios listed above

1. Skip the empty table while concatenating partial tables from batches
2. Error out with a CUDF_EXPECTS