prevent errors in the json encode process

[nullvariable]

adds an exception handler to the json.encode options so that the parser prints the name of the function or the reason it couldn't parse a key instead of crashing out.

Example before:

SCRIPT ERROR: type 'function' is not supported by JSON
[       script:test] 
[       script:test] > libPrint (@ox_lib/imports/print/server.lua:33)
[       script:test] > info (@ox_lib/imports/print/server.lua:42)
[       script:test] > ref (@test/server/test.lua:27)

After:

script:test] [test] [INFO] {
[       script:test]     "number": 30,
[       script:test]     "string": "Los Angeles",
[       script:test]     "fthis": "function: 0000020E8ABDD3F0"
[       script:test] }

Optionally returning nil instead of reason would still throw an error, while allowing objects with functions on them to be dumped in without needing to remove them first.