python plugin: Don't create empty meta_data_t objects.

Fixes: #716
src/utils_format_json.c: Handle empty meta_data_t gracefully.

Previously, `meta != NULL` and `keys_num == 0` was possible and would
result in "{}", which is not valid JSON.

Fixes: 716
src/meta_data.c: If the meta_data_t is empty, avoid calling malloc(0).
Merge remote-tracking branch 'github/collectd-5.3' into collectd-5.3