[mediacontroller][common] Support for null values in Json-Bundle conversions.

[mediacontroller][common] Support for null values in Json-Bundle conversions.

JsonToBundle and BundleToJson functions didn't support null values. Manually
handling them every time was cumbersome, hence this commit adds support for null values.

[Verification] + bundle c++ unittests 100% pass
+ bundle js tests 100% pass
+ tct-mediacontroller-tizen-tests 20190722 tct package 100% pass.

Signed-off-by: Michal Michalski <m.michalski2@partner.samsung.com>
Change-Id: I1315899d8ebb66935ea9b10791b5dba82341af45