json: allow empty string values

json: allow empty string values

The JSON standard permits empty strings. Currently fio's JSON module
returns an error when it encounters an empty string. Change this to
allow empty strings.

https://www.json.org/ is the reference I used for the JSON standard.

This is needed for upcoming patches to support FIO_OPT_STR_SET job
options (norandommap, time_based, etc) that are enabled when the option
is included without specifying a value for it. The value for these
options is best represented by an empty string. Disallowing empty string
values would prevent these options from appearing in job option
dictionaries in the JSON output.

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>