fix: Fixed a bug where serialize didn't write out empty vectors and mappings

[johan-gson]

Fixed a bug where serialize didn't write out empty vectors and mappings in a correct way. I have not added a test case, but I tested it in my code and it behaved as expected. Adding a test case at some point would be good.

Regarding the test cases, I couldn't get that to run when I followed the instructions, and don't have time to dig into why. I did test it using my own code though, and it seems to work.

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[fktn-k]

Looks good to me. Thanks!

[fktn-k]

Related issue: #454.